U-Pb zircon and monazite geochronology

In the Cree Lake Zone of northern Saskatchewan, reworked Archean orthogneisses are overlain by a highly deformed supracrustal sequence, the Paleoproterozoic Wollaston Group. This package of rocks was deformed and metamorphosed during the ca. 1.8 Ga continent–continent collision of the Trans-Hudson Orogen (THO), forming the Wollaston fold–thrust belt that underlies the eastern Athabasca Basin. The Hudsonian structural, metamorphic, and magmatic evolution of the Wollaston fold-thrust belt in the eastern Athabasca area involved six major stages. (1) Early collisional stage, DP1 at 1860–1835 Ma, involved burial of Wollaston Group metasediments from surface to depths equivalent to 3–5vkbar (1 kbar = 100 MPa) by thrust-pile stacking or imbrication tectonics, prograde metamorphism with garnet growth and development of early leucosomes, and emplacement of ca. 1840 Ma grey granite suite. (2) Collisional stage, DP2a at 1835–1820 Ma, involved continued deeper burial of Wollaston Group metasedi...

The Tasiast gold deposits are hosted within Mesoarchean rocks of the Aouéouat greenstone belt, Mauritania. The Tasiast Mine consists of two deposits hosted within distinctly different rock types, both situated within the hanging wall of the west-vergent Tasiast thrust. The Piment deposits are hosted within metasedimentary rocks including metaturbidites and banded iron formation where the main mineral association consists of magnetite-quartz-pyrrhotite ± actinolite ± garnet ± biotite. Gold is associated with silica flooding and sulphide replacement of magnetite in the turbidites and in the banded iron formation units. The West Branch deposit is hosted within meta-igneous rocks, mainly diorites and quartz diorites that lie stratigraphically below host rocks of the Piment deposits. Most of the gold mineralisation at West Branch is hosted by quartz-carbonate veins within the sheared and hydrothermally altered meta-diorites that constitute the Greenschist Zone. At Tasiast, gold mineralisation has been defined over a strike length >10 km and to vertical depths of 740 m. All of the significant mineralised bodies defined to date dip moderately to steeply (45º to 70º) to the east and have a south-southeasterly plunge. Gold deposits on the Tasiast trend are associated with second order shear zones that are splays cutting the hanging wall block of the Tasiast thrust. An age of 2839 ± 36 Ma obtained from the hydrothermal overgrowth on zircons from a quartz vein is interpreted to represent the age of mineralisation.

The three Himalayan lithologic units, the Lesser Himalayan Sequence, the Greater Himalayan Crystalline complex, and the Tethyan Himalayan Sequence, have a specifi c structural correlation with the Main Central thrust and South Tibet detachment in the central Himalaya. There, the Main Central thrust places the Greater Himalayan Crystalline complex over the Lesser Himalayan Sequence, and the South Tibet detachment places the Tethyan Himalayan Sequence over the Greater Himalayan Crystallines. Although this division has formed the basis for all Himalayan tectonic models, it fails to explain aspects of the geology of the western Himalaya where the Main Central thrust places the Tethyan Himalayan Sequence directly above the Lesser Himalayan Sequence. Our mapping in NW India shows that this relationship results from southward merging of the Main Central thrust and South Tibet detachment. This fi nding, in conjunction with observed alternating shear senses on the South Tibet detachment, is inconsistent with the wedgeextrusion and erosion-induced channel-fl ow models (both require only top-to-theN motion on the South Tibet detachment) but is consistent with a tectonic-wedging model.

The Gol-e-Gohar iron deposit in the Sanandaj-Sirjan zone of southwestern Iran comprises six major ore bodies. The largest deposit is Gol-e-Gohar No. 3 (Gohar-Zamin) with about 643 Mt @ 53.1% Fe. Magnetite is formed in massive and brecciated shapes. Gol-e-Gohar magnetite contains Mg, Ca, and Si up to the percent range, V and Ti in the 100s ppm level, and low Cr, Co, Ni in the tens of ppm range, typical of Kiruna mineralization (especially Bafq mining district). But Chador-Malu magnetite is formed at a higher temperature than Gol-e-Gohar magnetite, therefore, both deposits have the magmatichydrothermal high-T nature (magmatic ore-forming fluids), which are related to felsic magmatism (host meta-granites), and both of them are attributed to the Early Paleozoic. The oxygen isotope composition of magnetite is 4.9 ± 0.7‰ δ 18 O (n = 9) and the iron isotope composition is 0.49 ± 0.05‰ δ 56 Fe (n = 17). These data suggest that the magnetite ore formed from a magmatic-hydrothermal (high-T) fluid in equilibrium with a granitic source. The Gol-e-Gohar iron ore district shows several similarities to the Bafq mining district, located about 400 km to the north, and seems to be a disrupted member of the Kashmar-Kerman arc. Finally, according to the mentioned evidence and comparison of Gol-e-Gohar iron deposit with global samples, the genesis of mineralization in this deposit is most similar to Kirunatype (Kiruna-type magnetite ± apatite mineralization).

The Mesa Central of Mexico (MC) is an elevated plateau located 2000 m above sea level in central Mexico, where intrusions outcrop that register the history of exhumation-erosion occurring during the Late Cretaceous-Paleogene. The tectonic history of the region records formation of the Late Cretaceous-Paleogene 'Mexican orogen'; this was followed by extension of the entire region and several plutons were then exhumed. The age and magnitude of the crustal uplift and erosion occurring during exhumation has not been addressed to date. Therefore, this study reports the crystallization and cooling ages of two plutons, the Tesorera Granodiorite and the Comanja Granite, and estimates their emplacement depths. Based on these data, the exhumation age of the Tesorera Granodiorite is estimated to be between~73 Ma and~63 Ma at an exhumation rate of~528 m/m. y. and that of the Comanja Granite is 52 Ma and 48 Ma at an exhumation rate of~2500 m/m. y. Exhumation-erosion event of the Tesorera Granodiorite was located on the trace of the San Luis-Tepehuanes Fault System and that of the Comanja Granite on the a trace of the El Bajío Fault System. Furthermore, the high exhumation rate in the Comanja Granite suggests that gravitational collapse played an important role during exhumation.

The formation of accessory allanite, monazite and rutile in amphibolite facies rocks across the Barrovian sequence of the Central Alps (Switzerland) was investigated with a combination of petrography and geochemistry and related to the known structural and metamorphic evolution of the Lepontine dome. For each of these minerals a specific approach was adopted for geochronology, taking into account internal zoning and U-Th-Pb systematics. In-situ U-Th-Pb dating of allanite and monazite by ion microprobe revealed systematic trends for the ages of main deformation and temperature in the Lepontine dome. Isotope dilution TIMS dating of rutile returns dates in line with this picture, but is complicated by inheritance of pre-Alpine rutile and possible Pb loss during Alpine metamorphism. Allanite is generally a prograde mineral that is aligned along the main foliation of the samples and found also as inclusions in garnet. Prograde allanite formation is further documented by rutile inclusions with formation temperatures significantly lower than the maximum T recorded by the rock mineral assemblage. Allanite ages vary from 31.3 ± 1.1 Ma in orthogneisses in the East to 31.7 ± 1.1 Ma for a Bündnerschiefer and 28.5 ± 1.3 Ma for a metaquartzite in the central area, to 26.8 ± 1.1 Ma in the western part of the Lepontine dome. These ages are interpreted to date the main deformation events (nappe stacking and isoclinal deformation of the nappe stack), close to peak pressure conditions. The timing of the thermal peak in the Lepontine dome is recorded in monazite that grew at the expense of allanite and after a main episode of garnet growth at temperatures of ~620°C. Monazite in the central area yields an age of 22.0 ± 0.3 Ma, which is indistinguishable from the age of 21.7 ± 0.4 Ma from a metapelite in the

Monazite petrogenesis in the Nelson contact aureole is the result of allanite breakdown close to, but downgrade and therefore independent of, major phase isograds involving cordierite, andalusite and staurolite. The development of garnet downgrade of the staurolite and andalusite isograds does not appear to affect the onset of the allanite-to-monazite reaction but does affect the textural development of monazite. In lower pressure, garnet-absent rocks, allanite breakdown results in localized monazite growth as pseudomorphous clusters. In higher pressure, garnet-bearing rocks, allanite breakdown produces randomly distributed, lone grains of monazite with no textural relationship to the original reaction site. Fluids liberated from hydrous phases (chlorite, muscovite) during garnet formation may have acted as a flux to distribute light rare earth elements more widely within the rock upon allanite breakdown, preventing the localized formation of monazite pseudomorphs. Despite these textural differences, both types of monazite have very similar chemistry and an indistinguishable age by electron microprobe chemical dating (157 ± 6.4 Ma). This age range is within error of isotopic ages determined by others for the Nelson Batholith. Garnet from the garnet, staurolite and andalusite zones shows euhedral Y zoning typified by a high-Y core, low-Y collar and moderate-Y annulus, the latter ascribed to allanite breakdown during garnet growth in the garnet zone. The cause of the transition from high-Y core to low-Y collar, traditionally interpreted to be due to xenotime consumption, is unclear because of the ubiquitous presence of xenotime. Accessory phase geothermometry involving monazite, xenotime and garnet returns inconsistent results, suggesting calibration problems or a lack of equilibration between phases.

The Cordillera Darwin metamorphic complex is unique in the Andes in exposing kyanite-staurolite schist north of the Beagle Channel in southern Patagonia. Garnet in amphibolite facies pelitic schists from Bahı´a Pia has patchy textures whereby some grains consist of clear, grossular-rich garnet with fine-grained S1 inclusion trails truncated by regions of turbid spessartine-pyrope-rich garnet with biotite, muscovite, plagioclase and quartz inclusions. Micron-scale aqueous inclusions in turbid garnet are consistent with recrystallization facilitated by fluid ingress; S2 inclusion trails indicate this was broadly contemporary with the growth of kyanite and staurolite in the matrix. Pseudosection modelling in Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3-SiO 2-H 2 O-TiO 2-Fe 2 O 3 (NCKFMASHTO) is used to infer a P-T path dominated by decompression from 12 to 9 kbar at T 620°C, coupled with garnet mode decreasing from 5% to <1%. U-Th-Pb in situ dating of S2 monazite indicates that staurolite and kyanite growth and thus exhumation was underway before 72.6 ± 1.1 Ma. Contact aureoles developed adjacent to late granite intrusions include sillimanite-bearing migmatites formed at P 6 kbar after 72 Ma. Metamorphism of southern Cordillera Darwin induced by continental underthrusting beneath the arc, related to closure of the Rocas Verdes back-arc basin, was terminated by thrusting-controlled exhumation, with the rocks at P 9 kbar by c. 73 Ma and 6 kbar by c. 70 Ma.

This study makes a comparison between the populations of detrital zircon of the Cambrian sandstones from the Ossa-Morena Zone (OMZ) and the Ordovician quartzites from the southern domains of the Central Iberian Zone (S-CIZ) in order to identify the sources during development of North Gondwana basins (SW Iberia). The U-Pb results obtained for the Lower Cambrian sandstones of the OMZ show a remarkable similarity with the detrital zircon ages of greywackes from the underlying OMZ Ediacaran basement (Série Negra Succession). However, there is a greater proportion of the Cryogenian grains in the Cambrian rocks which main sources are: i) the Late Cadomian magmatic arcs (Ediacaran, ca. 635-545 Ma) which also contributed to infill the Late Ediacaran basins of the OMZ; and ii) the Early Cadomian arcs (Cryogenian, ca. 700-635 Ma). In the Lower Ordovician quartzites of the S-CIZ (Armorican and Sarnelha formations) the age distribution of detrital zircons overlaps the population of detrital zircons of the underlying S-CIZ Ediacaran basement (Beiras Group). Nevertheless, there are some differences in the Sarnelhas quartzites which have a population of detrital zircons similar to those of the Ediacaran greywackes and Cambrian sandstones of the OMZ. The Cambrian grains found in the Lower Ordovician quartzites fit the ages of magmatism representing the onset of rifting in North Gondwana that occurs in the OMZ and is absent in the S-CIZ. The Lower Ordovician grains are probably related to the magmatic event that preceded the passive margin stage of the Rheic Ocean, and exist in the CIZ and OMZ.

The determination of U-Pb ages from detrital zircons of sedimentary rocks using LA-ICP-MS has been widely used to develop studies of provenance analysis. A problem that frequently arises is to find a population that appears to be noncomplex despite several perceptible age peaks in their spectrum. These peaks are qualitatively defined through diagrams of relative probability (probability density function or PDF), but it is difficult to quantify their statistical significance relative to a zircon-forming multipeak event. Therefore, the question arises as to whether we can decipher and characterize a multipeak event in a noncomplex set of detrital zircon U-Pb ages. This work is an attempt to answer the above question by means of a statistical analysis. The objectives are: (1) to determine the most appropriate minimum number of zircon age populations (peaks); (2) to characterize each peak in terms of age and event duration; and (3) to compare results obtained for two datasets showing similar zircon ages. The process starts by using a cluster analysis to group zircon ages into a set of consistent clusters. A Gaussian kernel function is then fitted to each cluster and summed to obtain a theoretical PDF. At the end of the process, the best modelled PDF must coincide with the original PDF at C95 %, and the deciphered peaks can then be characterized.

Early Cambrian volcanic rocks of the Posht-e Badam tectonic block in Central Iran which is hydrothermally altered to green color rocks, hosts the Kiruna-type Iron Oxide-Apatite (IOA) mineralization in Anomaly 10 of Sechahun deposit. Geochemically, these volcanic rocks are classified as high-potassium calc-alkaline and mainly consist of rhyolite and trachyandesite. These rocks are important for hosting the REE-bearing apatite-iron oxide ores. The rhyolitic tuffs with a likely common origin are associated with the volcanic rocks. In this research, a selection of samples from Anomaly 10 of Sechahun has been studied with respect to their geochemistry and petrography with a special focus on REE-bearing phases. The bulk geochemistry of the rock units was analyzed using (ICP-OES) and (ICP-MS) methods. The ore samples were studied using ore microscopy and scanning electron microscopy with the energy-dispersive X-ray spectroscopy (SEM-EDS) techniques. The results indicated that mineralization has occurred by magmatic differentiation, forming magnetite, hematite, and apatite, followed by hydrothermal reworking, forming minerals such as hematite ± actinolite. Dendritic forms of magnetite, and apatite are common in the area. Apatite grains with monazite inclusions are generally enriched in LREE. The similar distribution of trace elements in various types of volcanic rocks in Anomaly 10 indicates a co-genetic origin and possible petrogenetic relation of these rocks. The whole rocks have been derived from differentiation of a single magmatic source. This magma with a probable melasyenitic initial composition erupted in a rifting tectonic environment and experienced some partial melting of the crust.

The Western Nyainqentanglha (WNT) mountain range of south-central Tibet predominantly comprises granitoids that intrude into metasedimentary strata, and was exhumed from~15 to 20 km depth in the footwall of the Yangbajain graben during the late Neogene. The range provides a rare exposure of deeper crustal levels of Tibet, which can be used to gain insight into processes that may be occurring beneath the plateau. Field, petrological, thermobarometric and U-Pb geochronological analyses are applied to determine the magmatic and metamorphic history of the WNT, revealing three tectonothermal events: (1) 213-201 Ma magmatism and amphibolite-facies metamorphism associated with north-south Lhasa terrane accretion, (2) 140-52 Ma magmatism resulting from subduction of Neotethys preceding India-Asia collision, and (3) 25-8 Ma magmatism that we suggest to have resulted from partial melting of the thickened Tibetan plateau crust. The latter is correlated with seismic bright spots imaged in the region at~15-18 km depth, indicating that restricted partial melting at mid-crustal levels may have been ongoing since 25 Ma, in accordance with observations from xenolith data and predictions made by thermal modelling of thickened crust.

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We present zircon and titanite U-Pb ages from granulite-facies transtensional shear zones and a greenschist facies strike-slip shear zone in the Karkkila-Vihti area, West Uusimaa. The zircon rim and titanite ages of c. 1815−1820 Ma from the transtensional shear zones constrain the evolution of the orogenic deformation from compressiondominated to syn-compressional lateral-escape within this part of the Fennoscandian shield while the zircon rim and titanite ages of 1790−1800 Ma give the timing of the late-to post-orogenic low-grade reactivation of the large-scale Somero-Karkkila fault zone. The new age data further constrain the timing of the changes in the crustal deformation style during orogenesis from i) compression-dominated to combined transpression-lateral stretching and escape, to ii) the late-to post-orogenic transition into retrograde conditions.

A study of the Jean Lake area of the Quetico metasedimentary belt, Superior Province, Canada, was conducted in order to evaluate the origin, source and evolution of sedimentary rocks, including mafic rocks previously mapped as ultramafics rocks. Bulk chemical compositions of these rocks show a mixing with two end members: quartzo-feldspathic sediments and komatiitic basalts. High CaO and MgO contents of the rocks suggest a proximal source of komatiitic basalts, probably from the southern Wabigoon subprovince. The rocks in the Jean Lake area record a pressure-temperature (P-T) path with three tectonometamorphic stages. The first stage formed staurolite (500-700°C) under medium P-T (MP-MT) metamorphic conditions shortly after the sedimentation. The second stage yielded the biotitesillimanite-garnet assemblage under the peak condition of 0.6  0.1 GPa and 700  70°C during transpressional deformation. The third stage, low P-medium T (0.25  0.11 GPa, 540  80°C) metamorphism, was associated with regional south-southeast compression and its timing is constrained by a new U-Th-Pb monazite age of 2667  20 Ma. Combining the regional deformation events, we suggest that the sediments in the Jean Lake area were buried up to MP-MT conditions during the D1 deformation at 2698-2689 Ma. They attained the peak metamorphic condition during the regional transpressive D2-D3 deformation (2689-2671 Ma), and retrograded to LP-MT condition during the south-southeast compression of the regional D4 at 2671-2667 Ma. The tectono-metamorphic history of the study area is consistent with the sedimentation in an accretionary prism followed by its docking to the Wabigoon subprovince to the north and Wawa greenstone belt to the south. The P-T-time path of this Archean accretionary prism is similar to that in modern arc accretion systems, except for a higher temperature gradient, ~ 30°C/km, recorded in the Quetico belt compared to ~10°C/km in modern counterparts. The high temperature gradients in the Quetico belt explain the lack of high-pressure metamorphic rocks, such as blueschist, that are common in modern accretionary prisms.

The Mogok Metamorphic Belt (MMB), situated in central Myanmar, contains an assemblage of highgrade metamorphic rocks believed to have been formed during a regional Early Eocene-Oligocene metamorphic event. We newly found Grt-Opx granulite in the Mogok area, which was formed under pressuretemperature (P-T) conditions estimated as being 6.5-8.7 kbar and 800-950 °C. Based on the results of EPMA U-Th-Pb Mnz dating, the central MMB, which runs through the Mogok, Mandalay, and Meikthila areas, is characterized by an Eocene to Oligocene deformation and fluid infiltration event. In addition, several Mnz grains from the Meikthila and Mandalay areas record ages of the Late Triassic (~ 200 Ma) and Cretaceous (~ 80 Ma and 110 Ma) geologic events.

The occurrence and relative stability of metamorphic allanite and monazite in metapelites of the Greater Himalayan Sequence (GHS) in the Dhauliganga valley, Garhwal Himalaya have been studied. The GHS is marked by kyanite grade metamorphism in the lower and middle structural levels and sillimanite grade metamorphism in the upper structural level. The occurrence of allanite is restricted to the lower structural level, while monazite occurs in the middle and upper structural levels. The allanite-out reaction occurred at ~660 o C and 9.5 kbar. Monazite shows widely varying ThO 2 content which can be explained mostly by brabantite substitution. The rocks of the GHS of the Dhauliganga valley display inverted metamorphic sequence and the allanite-out reaction is the result of prograde metamorphism.

The advent of a large amount of more precise U-Pb age data on Variscan granitoids from NW Iberia in recent years has provided a more focused picture of the magmatic history of the Western European Variscan belt (WEVB). Based on these data, three main pulses of magmatic activity seem to be well established.

The iron oxide-apatite (IOA) deposits of the eastern Adirondack Mountains consist of intrusive sheets or dikes of magnetite, fluorapatite, augitic pyroxene, quartz, and microcline. Other trace mineral phases include ilmenite with hematite exsolution, Vrich titanite rimming magnetite, zircon, monazite-(Ce), stillwellite-(Ce), lanthanite-(Ce), allanite-(Ce), and thorite. Observations under transmitted light show polygonal and cumulate textures. The ore bodies, each with knife-edge contacts with the host gneisses, are closely associated in time with A-type leucogranites and granitic gneisses (ca. 1070-1050 Ma). Backscattered electron (BSE) images highlight the following types of fluorapatite-monazite-(Ce) relations that formed as a result of metamorphism and fluid-rock interaction: (1) areas of relatively low BSE intensities containing tiny secondary monazite-(Ce) and thorite crystals developed within brighter apatite grains and along crystal margins and fractures; (2) areas of low BSE intensity within larger fluorapatite grains; (3) oriented rods of quartz in fluorapatite; (4) monazite-(Ce) rimming fluorapatite; and (5) multi-domain clusters of fluorapatite in unzoned fluorapatite. Fluorapatite samples from these IOA deposits were analyzed by electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) for major and trace elements. All grains contain high concentrations of light and heavy rare earth elements and yttrium, and samples from some deposits are exceptionally enriched in heavy rare earth elements and yttrium. The crystal structures of the fluorapatites from Cheever, Mineville, Palmer Hill, Arnold Hill, and Rutgers mine were analyzed. Rare earth elements (REE) are dominant at the Ca2 site, and in the most REE-rich sample, from Cheever, 5.7% of the Ca2 sites and 3.5% of the Ca1 sites are occupied by REEs. We consider that the most likely geological scenario for the incorporation of the REEs in fluorapatite includes high concentrations of incompatible elements in a dense iron-and phosphorus-rich melt that formed by immiscibility with the silicate melt of the Lyon Mountain granite. The high REE concentration appears to have been accommodated in the fluorapatite structure through a coupled substitution with Si 4þ. A later, low-temperature stage of fluid infiltration, probably at greenschist facies conditions, re-mobilized the REE and produced secondary minerals within the ore, including low-actinide bearing monazite-(Ce), tremolite, ferro-actinolite, chlorite, rutile, and hematite.

The Proterozoic belts that occur along the margins of the West Australian Craton, as well as those in intraplate settings, generally share similar geological histories that suggest a common plate-margin driver for orogeny. However, the thermal drivers for intraplate orogenesis are generally more poorly understood. The Mutherbukin Tectonic Event records a protracted period of Mesoproterozoic reworking of the Capricorn Orogen and offers significant insight into both the tectonic drivers and heat sources of long-lived intraplate orogens. Mineral assemblages and tectonic fabrics related to this event occur within a 50 km-wide fault-bound corridor in the central part of the Gascoyne Province in Western Australia. This zone preserves a crustal profile, with greenschist facies rocks in the north grading to upper amphibolite facies rocks in the south. The P-T-t evolution of 13 samples from 10 localities across the Mutherbukin Zone is investigated using phase equilibria modelling integrated with in situ U-Pb monazite and zircon geochronology. Garnet chemistry from selected samples is used to further refine the P-T history and shows that the dominant events recorded in this zone are prolonged D 1 transpression between c. 1320 and 1270 Ma, followed by D 2 transtension from c. 1210 to 1170 Ma. Peak metamorphic conditions in the mid-crust reached >650 °C and 4.4-7 kbar at c. 1210-1200 Ma. Most samples record a single clockwise P-T evolution during this event, although some samples might have experienced multiple perturbations. The heat source for metamorphism was primarily conductive heating of radiogenic mid-and upper crust, derived from earlier crustal differentiation events. This crust was thickened during D 1 transpression, although the thermal effects persisted longer than the deformation event. Peak metamorphism was terminated by D 2 transtension at c. 1210 Ma, with subsequent cooling driven by thinning of the radiogenic crust. The coincidence of a sedimentary basin acting as a thermal lid and a highly radiogenic mid-crustal batholith restricted to the Mutherbukin Zone accounts for reworking being confined to a discrete Accepted Article This article is protected by copyright. All rights reserved. crustal corridor. Our results show that radiogenic regions in the shallow to mid crust can elevate the thermal gradient and localize deformation, causing the crust to be more responsive to far-field stresses. The Mutherbukin Tectonic Event in the Capricorn Orogen was synchronous with numerous Mesoproterozoic events around the West Australian Craton, suggesting that thick cratonic roots play an important role in propagating stresses generated at distant plate boundaries.

The iron oxide-apatite (IOA) deposits of the eastern Adirondack Mountains consist of intrusive sheets or dikes of magnetite, fluorapatite, augitic pyroxene, quartz, and microcline. Other trace mineral phases include ilmenite with hematite exsolution, Vrich titanite rimming magnetite, zircon, monazite-(Ce), stillwellite-(Ce), lanthanite-(Ce), allanite-(Ce), and thorite. Observations under transmitted light show polygonal and cumulate textures. The ore bodies, each with knife-edge contacts with the host gneisses, are closely associated in time with A-type leucogranites and granitic gneisses (ca. 1070-1050 Ma). Backscattered electron (BSE) images highlight the following types of fluorapatite-monazite-(Ce) relations that formed as a result of metamorphism and fluid-rock interaction: (1) areas of relatively low BSE intensities containing tiny secondary monazite-(Ce) and thorite crystals developed within brighter apatite grains and along crystal margins and fractures; (2) areas of low BSE intensity within larger fluorapatite grains; (3) oriented rods of quartz in fluorapatite; (4) monazite-(Ce) rimming fluorapatite; and (5) multi-domain clusters of fluorapatite in unzoned fluorapatite. Fluorapatite samples from these IOA deposits were analyzed by electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) for major and trace elements. All grains contain high concentrations of light and heavy rare earth elements and yttrium, and samples from some deposits are exceptionally enriched in heavy rare earth elements and yttrium. The crystal structures of the fluorapatites from Cheever, Mineville, Palmer Hill, Arnold Hill, and Rutgers mine were analyzed. Rare earth elements (REE) are dominant at the Ca2 site, and in the most REE-rich sample, from Cheever, 5.7% of the Ca2 sites and 3.5% of the Ca1 sites are occupied by REEs. We consider that the most likely geological scenario for the incorporation of the REEs in fluorapatite includes high concentrations of incompatible elements in a dense iron-and phosphorus-rich melt that formed by immiscibility with the silicate melt of the Lyon Mountain granite. The high REE concentration appears to have been accommodated in the fluorapatite structure through a coupled substitution with Si 4þ. A later, low-temperature stage of fluid infiltration, probably at greenschist facies conditions, re-mobilized the REE and produced secondary minerals within the ore, including low-actinide bearing monazite-(Ce), tremolite, ferro-actinolite, chlorite, rutile, and hematite.

In the Grenville Province of western Quebec, Archean gneisses and Paleo-to Mesoproterozoic dykes have been reworked during a short-lived high-pressure metamorphic event at about 1.0 Ga. To the SE of the Grenville Front, a SE-dipping 30 kmthick slab of quartzofeldspathic granulites and biotite-garnet (±opx) gneisses overrides, along the McLaurin thrust, a 4 km-thick zone of steeply dipping muscovite-sillimanite micaschists that abuts on the Grenville Front. The granulitic slab in turn dips under amphibolite-grade migmatites, along the Dorval detachment. The micaschists, the granulites, the biotite-garnet gneisses, and the migmatites are of Archean provenance (Sm-Nd model ages) and correspond to the Grenvillian parautochthon. Metamorphosed mafic dykes are found to intersect at several localities the gneissic foliation of these rocks. Pods of coronitic metagabbros are also found in the same area. New U-Th-Pb chemical ages on monazite from the host rocks support previous estimations of ca. 2.7 Ga for the Archean metamorphism and 1.0 Ga for the Grenvillian reworking. As these determinations failed to recognize intermediate ages, it is likely that Proterozoic mafic dykes and pods are truly monocyclic, i.e. they only went through the Grenvillian metamorphic event. Metamorphic dykes have preserved fine-grained chilled margins, igneous fabrics, magmatic layering and Fe-Ti-rich, Si-poor chemistry in spite of a strong overprint attested by various types of reaction rims (coronitic structures). The coronitic gabbros lack magmatic fabrics and have a more primitive composition with higher Mg-number and normative quartz. In metamorphosed mafic dykes of the granulite slab, large pyroxenes are rimmed by garnet coronas with minute quartz inclusions, suggesting the reaction pl+opx = grt+qtz. Garnet coronas, in turn, are locally replaced by symplectites of opx, hbl, pl and qtz, probably formed through back reactions grt+cpx+qtz = pl+opx and grt+cpx = hbl+qtz. Thermobarometric calculations using the above assemblages indicate peak equilibration at 2 3 about 1.2-1.4 GPa at temperatures of about 800°C followed by a quasi-isothermal decompression in the vicinity of 0.9 MPa at temperatures around 700°C. Cross-cutting relationships and the persistence of magmatic fabrics within the dykes of the granulitic slab suggest emplacement in a brittle extensional setting whereas garnet coronas attest to subsequent burial under high-pressure (HP) granulite facies at relatively high temperature. As it is likely that some of the metamorphosed dykes belong to the 1.14 Gaold Abitibi swarm, Grenvillian HP granulite metamorphism would corresponds to burial of upper crustal levels down to sub-Moho depths with concomitant heating, followed by rapid exhumation. This situation is reminiscent of the emplacement-deformation-exhumation history of the SE-striking 1240 Ma-old Sudbury dyke swarm in Ontario (Bethune, 1997) and may be a characteristic of the Grenville Front Tectonic Zone.

We propose that the 495-470 Ma Ollo de Sapo magmatism in the Iberian Massif was the result of a mantle plume event in the Furongian-Early Ordovician. This plume was located beneath the northwestern margin of Gondwana and caused rapid and extensive melting of Ediacaran arcrelated crustal rocks and their derived sediments. Mafic magmas due to plume partial melting underplated the crust and were emplaced at mid-lower crustal levels (~15km), and provided the heat for crustal melting. Manifestations of the plume include (i) pronounced magnetic anomalies in the region which match the distribution of Ollo de Sapo rocks, (ii) the Toledanian unconformity (and gap), which is attributed to thermal doming, and (iii) the significant thickness variations in Lower to Middle Ordovician sedimentary successions in adjacent areas. In a more regional context, we infer that the plume was one of a cluster of plumes impacting the Gondwana periphery and that it contributed to the birth and development of the Rheic Ocean throughout Gondwana margin breakup.

A study of the Jean Lake area of the Quetico metasedimentary belt, Superior Province, Canada, was conducted in order to evaluate the origin, source and evolution of sedimentary rocks, including mafic rocks previously mapped as ultramafics rocks. Bulk chemical compositions of these rocks show a mixing with two end members: quartzo-feldspathic sediments and komatiitic basalts. High CaO and MgO contents of the rocks suggest a proximal source of komatiitic basalts, probably from the southern Wabigoon subprovince. The rocks in the Jean Lake area record a pressure-temperature (P-T) path with three tectonometamorphic stages. The first stage formed staurolite (500-700°C) under medium P-T (MP-MT) metamorphic conditions shortly after the sedimentation. The second stage yielded the biotitesillimanite-garnet assemblage under the peak condition of 0.6  0.1 GPa and 700  70°C during transpressional deformation. The third stage, low P-medium T (0.25  0.11 GPa, 540  80°C) metamorphism, was associated with regional south-southeast compression and its timing is constrained by a new U-Th-Pb monazite age of 2667  20 Ma. Combining the regional deformation events, we suggest that the sediments in the Jean Lake area were buried up to MP-MT conditions during the D1 deformation at 2698-2689 Ma. They attained the peak metamorphic condition during the regional transpressive D2-D3 deformation (2689-2671 Ma), and retrograded to LP-MT condition during the south-southeast compression of the regional D4 at 2671-2667 Ma. The tectono-metamorphic history of the study area is consistent with the sedimentation in an accretionary prism followed by its docking to the Wabigoon subprovince to the north and Wawa greenstone belt to the south. The P-T-time path of this Archean accretionary prism is similar to that in modern arc accretion systems, except for a higher temperature gradient, ~ 30°C/km, recorded in the Quetico belt compared to ~10°C/km in modern counterparts. The high temperature gradients in the Quetico belt explain the lack of high-pressure metamorphic rocks, such as blueschist, that are common in modern accretionary prisms.

In 1974, BATAN conducted a survey to review the potential of radioactive minerals in Harau, West Sumatra. The survey shows that there are radiometric anomalies in several locations in the area. However, the survey has not been able to show the radioactive elements and minerals found in Harau. This study aims to identify and characterize radioactive elements and minerals in Harau. To prove that detailed geological mapping, radiometric mapping with Gamma Spectrometer RS-125, and analysis using Micro-XRF &quot;M4 Tornado Plus®&quot;. The results of geological and radiometric mapping show that there are high thorium contents in the metamorphic rocks of the Kuantan Formation reaching 2300 ppm eTh. Identification of thorium-bearing minerals using a micro-XRF instrument resulted in thorium-bearing minerals found in the Harau area are thorite (ThSiO4), yttrialite (YThSi2O7), and monazite ((Ce,Nd,Th)PO4) with thorium levels within 2.75 - 42.75% ThO2 but experiencing a significant increase in...

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Geological 3D modeling delivers essential information on the distribution of enrichment zones and structures in (complex) mineral deposits and fosters a better guidance to subsequent exploration stages. The Paleoproterozoic Epembe carbonatite complex showcases the close relation between enrichment of specific elements (Nb, Ta, P, Total Rare Earth Element (TREE) + Y) and shear zones by structural modeling combined with geochemical interpolation. Three-dimensional fault surfaces based on structural field observations, geological maps, cross-sections, and drillhole data are visualized. The model shows a complex, dextral transpressive fault system. Three-dimensional interpolation of geochemical data demonstrates enrichment of Nb, Ta, P, and TREE + Y in small, isolated, lens-shaped, high-grade zones in close spatial distance to faults. Based on various indicators (e.g., oscillating variograms, monazite rims around the apatite) and field evidence, we see evidence for enrichment during hyd...

The apatite assemblage from Maglovec hill (Slanské vrchy Mountains near the city of Prešov) from fissures of hydrothermally altered neovolcanic rocks (andesites and related lithologies) was studied. The assemblage consists of two different morphological apatite types (apatite in cores of prismatic crystals and fibrous apatite mantling these cores). The assemblage was investigated by a multi-analytical approach to reveal its unique chemical composition and structure. Both types of apatite display zoning visible in back-scattered electron (BSE) images. Core apatite is relatively homogenous with porous rims appearing darker in the BSE images at the contact with fibrous apatite, and occasionally with darker regions along fractures. These parts are depleted in trace elements, mostly in LREE. Fibrous apatites display concentric and/or patchy zoning. Dark regions in fibrous apatite occasionally display a porous structure. In part of fibrous crystals, substitution of (CO3)2− for phosphorus ...

The Precambrian Taratash complex (Middle Urals) is one of the rare windows into the Palaeoproterozoic and earlier history of the eastern margin of the East European Craton. Monazite from intensively deformed rocks within a major amphibolite-facies shear zone in the Taratash complex has been investigated by means of electron-probe microanalysis and laser-ablation SF-ICP-MS. Metamorphic and magmatic cores of monazite from metasedimentary and metagranitoid rocks yield U-Pb ages of 2244 ± 19 and 2230 ± 22 Ma (± 2 σ) and record a previously unknown pre-deformational HTmetamorphic event in the Taratash complex. Subsequent dissolution-reprecipitation of monazite, during shear zone formation under amphibolite-facies conditions, caused patchy zonation and chemical alteration of the recrystallised monazite domains, leading to higher cheralite and huttonite components. This process, which was mediated by a probable (alkali + OH)-bearing metamorphic fluid also caused a total resetting of the U-Pb-system. The patchy domains yield concordant U-Pb-ages between 2052 ± 16 and 2066 ± 22 Ma, interpreted as the age of the shear zone. In line with previously published ages of high grade metamorphism and migmatisation, the data may point to a Palaeoproterozoic orogenic event at the eastern margin of the East European Craton. Post-deformational fluid-induced greenschist-facies retrogression caused partial to complete breakdown of monazite to fluorapatite, REE + Y-rich epidote, allanite and Th-orthosilicate.The retrograde assemblages either form coronas around monazite, or occur as dispersed reaction zones, indicating that the REE, Y, and Th were mobile at least on the thin section scale. The greenschist-facies metamorphic fluid was aqueous and rich in Ca. Monazite affected by advanced breakdown responded to the retrogression by incorporating the cheralite or huttonite components during a fluid-induced dissolution-reprecipitation process. This event did not reset the U-Pb-system but caused partial Pb loss reflected by discordant U-Pb-dates.

The Mogok Metamorphic Belt (MMB), situated in central Myanmar, contains an assemblage of highgrade metamorphic rocks believed to have been formed during a regional Early Eocene-Oligocene metamorphic event. We newly found Grt-Opx granulite in the Mogok area, which was formed under pressuretemperature (P-T) conditions estimated as being 6.5-8.7 kbar and 800-950 °C. Based on the results of EPMA U-Th-Pb Mnz dating, the central MMB, which runs through the Mogok, Mandalay, and Meikthila areas, is characterized by an Eocene to Oligocene deformation and fluid infiltration event. In addition, several Mnz grains from the Meikthila and Mandalay areas record ages of the Late Triassic (~ 200 Ma) and Cretaceous (~ 80 Ma and 110 Ma) geologic events.

One of the enduring debates in the study of the Himalayan orogen (and continental collision zones in general) is whether the salient observed features are explained (a) by localized deformation along discrete, narrow fault zones/ductile shear zones separating individual blocks or slices (e.g. critical taper or wedge tectonic models), or (b) by distributed deformation dominated by wide zones of visco-plastic flow in the solid or a partially molten state (e.g. channel flow models). A balanced cross section from Sikkim in the eastern Himalaya that is based on structural data and is drawn to satisfy petrological and geophysical constraints as well, is used in combination with information from petrology, geochronology, geospeedometry and microstructural data to address this question. We discuss that any tectonic model needs to be thermally, rheologically, geometrically and temporally viable in order to qualify as a suitable description of a system; models such as channel flow and critical taper are considered in this context. It is shown that channel flow models may operate with or without an erosional porthole (channel with tunnel and funnel mode vs. channels with only the tunnel mode) and that the predicted features differ significantly between the two. Subsequently, we consider a large body of data from Sikkim to show that a channel flow type model (in the tunneling without funneling mode), such as the ones of Faccenda et al., (2008), describes features formed at high temperatures very well, while features formed at lower temperatures are more consistent with the operation of localized, fault-bounded, slice tectonics, LFBST (be it in the form of critical taper, wedge tectonics, or something else). Thus, the two modes are not competing, but collaborating, processes and both affect a given rock unit at different points of time during burial, metamorphism and exhumation. A transitional stage separates the two end-member styles of tectonic evolution. The proposed models bear similarities to those suggested by Mallet (1875) and Auden (1935) and mechanisms proposed by Beaumont and Jamieson (2010). We conclude by discussing some

The lesser and greater Himalayan sequences (LH and GH) in Sikkim have been studied in detail recently. The LH consists mainly of pelitic lithologies with detrital zircon ages >1800 Ma and e Nd (0) of À27.7 to À23.4 (with intercalated calcsilicates, metabasites and a sheared granite gneissthe Lingtse gneiss). The metamorphic sequence is characterized by (i) a remarkably continuous and systematic metamorphic field gradient (~60-70°C kbar À1), (ii) peak pressures as well as temperatures of metamorphism increasing at higher metamorphic grades (~5 kbar, 500°C at the garnet zone tõ 8 kbar, 700°C at the sillimanite-muscovite-zone), (iii) clockwise P-T paths of a tight 'hairpin' shape with a prograde slope of 20-30°C kbar À1 , (iv) initiation of garnet growth (Lu-Hf chronometry) at systematically older ages at higher grades (11-10.5 Ma at garnet zone to 16.8 Ma at sillimanite-K-feldspar zone), (v) peak of metamorphism simultaneously at all grades (13-11 Ma), (vi) deformed and folded metapelitic rocks but with little evidence of pervasive shearing and (vii) duration of melt formation and segregation on the order of a few 100,000 years. The degree of melting was low, as indicated by the presence of muscovite all through the sequence. This evidence indicates that the LH behaved as a coherent block, and thermomechanical models quantitatively reproduce many of these features as well as explain the formation of inverted metamorphic sequence in such a coherent block. The greater Himalayan sequence (GH), also largely metapelitic, has different protoliths (with components as young as 800 Ma) and a different metamorphic history (extensive migmatization at higher grades~8-12 kbar, 750-850°C at an older age of largely >20 Ma along a different clockwise P-T path characterized by sharp, isothermal decompression followed by near isobaric cooling). Geochronological data (monazite, zircon, garnet) as well as cooling rate calculations (geospeedometry) reveal that the GH is made up of at least two blocks with distinct, complex and multistage P-T-t histories. Different stages of the high-T cooling were rapid (100's°C Ma À1 to 10's°C Ma À1 at different stages) and at least some of it was unrelated to exhumation; synthesis of these results with low-T thermochronometry allows the detailed multistage cooling and exhumation history of this block to be reconstructed. The reconstructed cooling and exhumation histories show that a number of interlinked processes operating on a hierarchy of timescales leads to the overall evolution. The P-T history with rapid cooling can be reproduced by thermomechanical models that consider conductive as well as advective heat transport and there is no need to invoke additional processes (such as the role of fluids). Metabasic inclusions within both the LH and the GH appear to record a different (? older in part) history. The role of these in the overall evolution, as well as the difference in metamorphic ages of the LH and GH remain open questions that need to be addressed in future research.

In SW Iberia there is a basal unconformity where Triassic sandstones overlie previously deformed Carboniferous turbidites. This important stratigraphic boundary records the transition from the final stages of Pangaea’s amalgamation to the initial stages of break-up. U-Pb dating of detrital zircon from Carboniferous greywackes of the South Portuguese Zone and Triassic sandstones of the Algarve and Alentejo basins was carried out to conduct a study of sedimentary provenance. The radiometric ages obtained by LA-ICP-MS show that most of the samples have common age spectra which are typical of North Gondwana sources: Archean and Paleoproterozoic ages characteristic of the West African craton, Neoproterozoic ages associated with Cadomian and Pan-African orogenies, and Cambrian ages related to the intra-continental rifting of North Gondwana. The only exception is a Visean immature greywacke that was probably derived from Devonian magmatic arcs related to the Variscan orogeny. Very rare pre...

The Morais Allochthonous Complex (MAC) belonging to the Galicia-Trás-os-Montes Zone, (GTMZ) is placed in the hinge zone of the Central Iberian Arc (CIA). Evidences of the pre-to syn-Variscan geological evolution of the NW Iberia are especially well exposed along the eastern rim of this first order structure, where this research work was achieved. Two major tectono-stratigraphic domains juxtaposed by a significant Variscan tectonic accident were redefined: the Schistose Domain of Galicia-Trás-os-Montes (SDGTM) and Central Iberian Zone (CIZ). The first domain belongs to the lowermost tectonic slice of the different imbricated allochthonous units of the GTMZ. The SDGTM was settled over the CIZ during the Variscan C2 stage by the Main Trás-os-Montes Thrust (MTMT), and it is considered to be the least allochthonous unit (also called Parautochthonous or Peritransmontano Domain). Since the pre-variscan stratigraphic records of both domains are very similar, it is assumed that they must have been attached to the same continental margin but in different palaeogeographic locations, as their detrital zircon age populations point out. Both domains present a quite continuous and traceable Cambro-Silurian (?) stratigraphic sequence, although it underwent more Variscan deformation in the SDGTM than in its relative autochthonous (CIZ). In the CIZ the identification of two ichnofossil genera in the Desejosa Formation (Teichichnus rectus and Rosselia socialis) confirms that its upper part is at least uppermost-early Cambrian (Cambrian Stage 3), as previously proposed near the studied region with badly preserved trilobite remains (522Ma). A new stratigraphic unit (Montes Ermos formation, informal) was found concordantly overlaying the Desejosa Formation and lying below the Lower Ordovician unconformity (or Toledanic Unconformity) meaning that the uppermost record of the Slate and Greywacke Complex in Iberia could reach ages younger than Cambrian Stage 3. Over the Toledanic unconformity, a quite complete Lower-Middle Ordovician sequence was mapped, showing a laterally continuous and condensed Armorican Quartzite facies. At the base of the Upper Ordovician limestones of the Santo Adrião Formation another unconformity was identified, eroding the underlying units towards the Lower Ordovician formations, which locally eliminates the Middle Ordovician record (Moncorvo Formation). The youngest studied unit of the autochthonous domain recognized in this work is Silurian, but due to the strong C2 shearing that affects it no further assumptions on its age should be made.

Attainment of high crustal heat fluxes and consequent partial melting is critical to the stabilization of continental roots. Understanding the processes and timescales behind partial melting of continental crust in the Archean is thus paramount for understanding Archean tectonic modes and how stable cratons formed. High-temperature (HT) to ultrahigh-temperature (UHT) metamorphic rocks can record evidence for dynamic processes that result in advective heat fluxes and a substantial deviation from normal crustal geothermal gradients. Examination of the pressure-temperature conditions and timescales of HT/UHT metamorphism is thus essential to understanding the tectonic processes behind extreme crust heat fluxes and the formation of stable cratonic crust. Here, utilizing both traditional and nontraditional petrologic and geochronologic techniques, we compare the pressure-temperaturetime paths of two Neoarchean terranes: the eastern Beartooth Mountains of the Wyoming Craton and the Pikwit...

A combined metamorphic and isotopic study of lit-par-lit migmatites exposed in the hanging wall of the Main Central Thrust (MCT) from Sikkim has provided a unique insight into the pressure-temperaturetime path of the High Himalayan Crystalline Series of the eastern Himalaya. The petrology and geochemistry of one such migmatite indicates that the leucosome comprises a crystallized peraluminous granite coexisting with sillimanite and alkali feldspar. Large garnet crystals (2-3 mm across) are strongly zoned and grew initially within the kyanite stability field. The melanosome is a biotite-garnet pelitic gneiss, with fibrolitic sillimanite resulting from polymorphic inversion of kyanite. By combining garnet zoning profiles with the NaCaMnKFMASHTO pseudosection appropriate to the bulk composition of a migmatite retrieved from c. 1 km above the thrust zone, it has been established that early garnet formed at pressures of 10-12 kbar, and that subsequent decompression caused the rock to enter the melt field at c. 8 kbar and c. 750°C, generating peritectic sillimanite and alkali feldspar by the incongruent melting of muscovite. Continuing exhumation resulted in resorption of garnet. Sm-Nd growth ages of garnet cores and rim, indicate pre-decompression garnet growth at 23 ± 3 Ma and near-peak temperatures during melting at 16 ± 2 Ma. This provides a decompression rate of 2 ± 1 mm yr)1 that is consistent with exhumation rates inferred from mineral cooling ages from the eastern Himalaya. Simple 1D thermal modelling confirms that exhumation at this rate would result in a near-isothermal decompression path, a result that is supported by the phase relations in both the melanosome and leucosome components of the migmatite. Results from this study suggest that anatexis of Miocene granite protoliths from the Himalaya was a consequence of rapid decompression, probably in response to movement on the MCT and on the South Tibetan detachment to the north.

New data related to the definition of the Oxfordian/Kimmeridgian boundary obtained mostly after 2006 are discussedi.e. after the proposal for the uniform boundary of the stages at the Flodigarry section at Staffin Bay on the Isle of Skye, northern Scotland has been given. This boundary is based on the Subboreal-Boreal ammonite successions-where it is distinguished at the base of the Subboreal Baylei Zone which is defined by the Pictonia flodigarriensis horizon, and which corresponds strictly to the base of the Boreal Bauhini Zone. This boundary is defined also by other stratigraphical data-both palaeontological, geochemical and palaeomagnetical (including its well defined position close to the boundary between magnetozones F3n, and F3r-the latter corresponding to marine magnetic anomaly M26r). The boundary and the section are thus proposed as the GSSP for the Oxfordian/Kimmeridgian boundary. The boundary is well recognizable also in other sections of Subboreal and Boreal areas as discussed in the study, including southern England,

tectonics continental collision Himalaya eclogite ultrahigh temperature crustal thickening Modern-style plate tectonics, often characterised by subduction, is a fundamental dynamic process for planet Earth. Subduction related eclogites are widely used to indicate initiation of plate tectonics or whether different tectonic regimes dominated Earth history. However, such markers are commonly overprinted in ancient metamorphic terranes and rarely preserved even in most Phanerozoic mountain belts. This study tries to reveal the detailed burial and exhumation processes that formed granulitized eclogites in the Everest east region, central Himalaya, so as to explore the tectonic regimes recorded by similar rocks on early Earth. Robust Pressure-Temperature-time paths were achieved by studying the mineral relicts (Omp, Jd ∼29%), high-temperature mineral textures (Sil-Crd-Qz-Spl-Mesoperthite assemblage, rutile exsolution in biotite), and multiple thermobarometry and petrochronology of eclogites and metapelites. Results show that these eclogites underwent eclogitization at conditions of 730-770 • C and ∼20 kbar (∼11 • C/km) at ∼30 Ma and were overprinted by a heating and decompression path to ultrahigh temperature (UHT) conditions of 6-11 kbar and 900-970 • C (∼40 • C/km) during 25-15 Ma. The resulting exhumation rate (2-3 mm/yr) is slow and prolonged (10-15 Myr) (U)HT favoured reequilibration of the eclogitic mineral assemblage and textures. The obtained UHT conditions, the first time ever reported for the Himalaya, were induced by combined effects of over-thickened (∼60 km) radioactive felsic crust and thinning of lithosphere to <90 km. This case study provides a critical example to understand the heat sources and timescale of UHT condition during continental collision. By comparing with the western Himalaya eclogites, we suggest that formation of cold vs. granulitized continental eclogites during the Himalayan orogeny is caused by different crustal thickness (normal ∼30 km vs. overthickened ∼60 km) due to different collisional stages (infant vs. mature). In a wider perspective, ancient eclogites were commonly granulitized by stacking into the over-thickened orogenic crust during mature continental collision. According to similar granulitized eclogites preserved on early Earth, Himalaya-type continental subduction/collision should have become a global pattern during the Paleoproterozoic (2.0-1.8 Ga).

The Mae Ping fault (MPF), western Thailand, exhibits dominantly left-lateral strike-slip motion and stretches for >600 km, reportedly branching off the right-lateral Sagaing fault in Myanmar and extending southeast towards Cambodia. Previous studies have suggested that the fault assisted the large-scale extrusion of Sundaland that occurred during the Late Eocene-Early Oligocene, with a geological offset of 120-150 km estimated from displaced high-grade gneisses and granites of the Chiang Mai-Lincang belt. Exposures of high-grade orthogneiss in the Lansang National Park, part of this belt, locally contain strong mylonitic textures and are bounded by strike-slip ductile shear zones and brittle faults. Geochronological analysis of monazite from a sample of sheared biotite-K-feldspar orthogneiss suggests two episodes of crystallization, with core regions documenting Th-Pb ages between c. 123 and c. 114 Ma and rim regions documenting a significantly younger age range between c. 45-37 Ma. These data are interpreted to represent possible magmatic protolith emplacement for the Lansang orthogneiss during the Early Cretaceous, with a later episode of metamorphism occurring during the Eocene. Textural relationships provided by in situ analysis suggest that ductile shearing along the MPF occurred during the latter stages of, or after, this metamorphic event. In addition, monazite analyzed from an undeformed garnet-two-mica granite dyke intruding metamorphic units at Bhumipol Lake outside of the Mae Ping shear zone produced a Th-Pb age of 66.2 ± 1.6 Ma. This age is interpreted to date the timing of dyke emplacement, implying that the MPF cuts through earlier formed magmatic and high-grade metamorphic rocks. These new data, when combined with regional mapping and earlier geochronological work, show that neither metamorphism, nor regional cooling, was directly related to strike-slip motion.

Fluorapatite with monazite-(Ce) and xenotime-(Y) microinclusions occurs in the lithium–caesium–tantalum pegmatite body A of the Volta Grande mine, Minas Gerais state, Southeast Brazil. The fluorapatite displays faint zoning, detected mainly by cathodoluminescence. Electron probe and laser ablation analyses indicate that zoning in the fluorapatite corresponds to variation in Mn and rare-earth element (REE) content. Such compositional variation is attributed to partial removal of the REE from the fluorapatite structure during a dissolution–reprecipitation process, forming monazite-(Ce) and xenotime-(Y) microinclusions in the REE-depleted zones of the fluorapatite. These inclusions exhibit an inherited geochemical signature, manifested by low Th and U concentrations when compared to monazite and xenotime crystallised from melts. Rhodochrosite and calcite inclusions are also associated with monazite-(Ce) and xenotime-(Y) and are probably products of the same process, recycling Ca, Mn, a...

A major arc-related magmatic episode is recorded in the Ediacaran-Early Cambrian (Cadomian) crust of Iran and Anatolia due to the southward subduction of Prototethyan oceanic lithosphere beneath N Gondwana. This magmatism was generated at a convergent margin, fragments of which can be traced for thousands of kilometres, from west Avalonia to the Lhasa terrane. Cadomian crustal tracts in Iran and Tauride-Anatolia are represented by abundant arc-like plutonic and volcanic rocks and a series of retro-arc, rifted basins dominated by thick sequences of terrigenous rocks as well as both calc-alkaline and alkaline magmatic rocks. This paper presents new zircon U-Pb as well as geochemical-isotopic data from plutonic (granite to gabbro) and volcanic (basalt to rhyolite) magmatic rocks of a Cadomian retro-arc rifted basin from Zarand in SE Iran. Geochemical data indicate two different geochemical signatures; high K calc-alkaline-shoshonitic, characterized by strong depletions in Nb, Ta, P, Ti, and alkaline rocks (and A 1-type intrusions), with strong OIB-like trace element patterns. Cadomian OIBlike rocks-as well as A 1-type intrusions-are rare in the basements of Iran and Anatolia and are only documented as minor gabbroic intrusions and/or (meta-) volcanic rocks from Saghand (central Iran), NE Iran and from exotic blocks from Ediacaran salt domes. Moreover, A 2-type granites are also present in the Cadomian crust of Iran. Zarand volcanic rocks are interlayered with Rizu-Dezu terrigenous sedimentary rocks, whereas plutonic rocks intruded these metasediments. New zircon U-Pb ages show that high K calc-alkaline-shoshonitic rocks formed 537-536 Ma, whereas A 1-type granites crystallized at~535 Ma. Most zircons from A 1-type granites show positive εHf(t) values from +1.1 to +5.1, mostly higher than high K calc-alkaline-shoshonitic rocks with εHf (t) between −6.6 and +8.1. Bulk rock Nd-Sr isotopic data (e.g., ɛNd(t)= +0.3 to +4.0) for OIB-like rocks confirm that these rocks originated from an enriched OIB-like mantle source, whereas high K calc-alkaline-shoshonitic rocks (with εNd(t) = −7.7 to −6.2) show strong interaction with, and/or re-melting of a continental crust. Our results are consistent with an interpretation that the Zarand volcanic-plutonic rocks as well as associated thick sequences of sedimentary strata rocks formed in a retro-arc rifted basin behind the Cadomian magmatic arc. This basin seems to have developed during the Ediacaran at~570-560 Ma with deposition of Ediacaran Morad to Lower Cambrian Rizu-Dezo sedimentary rocks. The retro-arc extensional basin seems to have become magmatically active at 540-535 Ma, as shown by the occurrence of both high K calc-alkaline-shoshonitic and OIB-like magmatic rocks. This basin was flanked by a cratonic hinterland towards Gondwana and a magmatic arc to the N. The cratonic hinterland fed detritus with detrital zircons older than 0.6 Ga into the basin, whereas the magmatic arc shed detritus with 500-600 Ma old zircons. Our compiled and new data from Ediacaran-Early Paleozoic magmatic rocks also show that Cadomian high magmatic fluxes occurred differently in parts of Iran-Anatolia, with overlaps in some areas.