Papers by Gaston Giuliani
Mineralium Deposita, 2009
The Ianapera emerald deposit is located in the Neoproterozoic Vohibory Block of southern Madagasc... more The Ianapera emerald deposit is located in the Neoproterozoic Vohibory Block of southern Madagascar. The local geology consists of intercalated migmatitic gneissic units and calcareous metasedimentary rocks, containing boudinaged metamorphosed mafic/ultramafic lenses, all intruded by pegmatite veins. These units occur near the hinge of the tightly folded Ianapera antiform, within a few kilometers of the Ampanihy shear zone. Emerald mineralization is hosted by metasomatic phlogopite veins, and bodies developed within the mafic/ultramafic rocks. Based on field and textural relationships, we distinguish proximal and distal styles of mineralization. Proximal mineralization occurs at the contact of pegmatite veins with mafic/ultramafic units; in the distal style, pegmatites are not observed. Three types of emeralds could be distinguished, mainly on the basis of color and mineral zoning. Some of these emeralds have the most Al-depleted and Cr-rich composition ever recorded. Another characteristic feature to the Ianapera deposit and, to our knowledge, yet unreported, is the association of some emeralds with scapolite in metasomatised mafic rocks. Mineral inclusions are common in most emeralds and include phlogopite, carbonates, barite, K-feldspar, quartz, pyrite, zircon, monazite, bastnaesite, phenakite, plus Fe and Cr oxides. However, feldspar and rare earth element-bearing minerals occur predominantly in proximal emeralds, which also have a more incompatible trace-element signature than distal emeralds. We propose a model related to syn-to post-tectonic magmatic-hydrothermal activity. Pegmatitic bodies intruded units of the Ianapera antiform probably during tectonic relaxation. Exsolution of fluids rich in halogens and incompatible elements from the cooling pegmatites caused hydrothermal metasomatism of Cr-bearing mafic/ultramafic rocks in direct contact with the pegmatites. Local fracturing favored fluid infiltration, permitting the formation of distal mineralization. Emerald composition was controlled by the chemistry of the host rock. The presence of carbonate mineral inclusions in the emeralds and the high F-activity indicated by elevated F-contents in newly formed minerals suggest transport of Be as a fluoridecarbonate complex. It seems likely that beryl formation was triggered by precipitation of F-rich phlogopite, which removed the complexing ligand from the fluid.
Journal of South American Earth Sciences, 2002
In the Guateque-Medina area, Paleozoic basement and Mesozoic rift basins have been uplifted and e... more In the Guateque-Medina area, Paleozoic basement and Mesozoic rift basins have been uplifted and exhumed during the Andean orogeny (12 Ma to present). Surface exposures and subsurface data constrain the deformation style and the rift geometry. We have mapped a regional transect and restored a cross section. We have also reconciled existing stratigraphic data, from cordillera, foothills and foreland basin, and have added new data of our own. In Early Cretaceous shales, there is evidence for fault-controlled sedimentation. A brecciated evaporitic layer, which is locally emerald bearing, has acted as a regional detachment. The underlying basement, composed of Paleozoic sediments, crops out as the Quetame Massif. It was uplifted during the Andean orogeny on a series of high-angle reverse faults. The main SE-verging Tesalia fault has resulted from Andean reactivation of an Early Cretaceous normal fault, which bounded a half-rift. A series of NW-verging back-thrusts may have resulted from Andean reactivation of Paleozoic faults. Between the back-thrusts and the Tesalia fault is a basement pop-up. It may be part of a flower structure, because components of right-lateral slip have been identified. These are attributed to eastward subduction of the Nazca plate beneath South America. In general, the style and timing of Andean deformation in the Guateque-Medina area are compatible with the plate tectonic setting of the northern Andes.
Geological Society, London, Special Publications, 1999
The emerald deposits of Muzo and Coscuez hosted within the western flank of the Colombian Eastern... more The emerald deposits of Muzo and Coscuez hosted within the western flank of the Colombian Eastern Cordillera formed during thrust tectonics affecting series of lower Cretaceous black shales. An unusual polygenetic breccia fills opened fractures and dilatant sites within ...
European Journal of Mineralogy, 2014
New electron-microprobe analyses of ‘tsavorites’ from the Neoproterozoic Metamorphic Mozambique B... more New electron-microprobe analyses of ‘tsavorites’ from the Neoproterozoic Metamorphic Mozambique Belt deposits allow the characterization of green grossular according to its trace-element chemistry (V, Cr, Mn). Five chemical types are defined: type 1, vanadian grossular with V >Cr >Mn (in atoms per formula unit); type 2, vanadian grossular with V >Mn >Cr; type 3, Mn-bearing vanadian grossular with Mn > V > Cr; type 4, Mn-bearing chromian grossular with Mn > Cr > V; and type 5, Cr- and Mn-bearing grossular with Cr > Mn > V. These types are also characterized by different absorption spectra in the ultraviolet–visible–near infrared. Type 1 tsavorite spectra show a total absorption below 430 nm due to the high vanadium contents. Type 2 tsavorite spectra present the classical absorption bands of V. Types 3 and 4 tsavorite spectra display additional shoulders at 407 and 408 nm due to Mn 2+ , whereas spectra of Cr-bearing types 4 and 5 tsavorite show the two additional bands of Cr 3+ at 697 and 701 nm. The different absorption spectra also indicate Fe 2+ -Ti 4+ charge transfer. We measured OH − equivalent to 0.08 to 0.38 wt% eq. H 2 O within the structure. Concentrations of vanadium, chromium and manganese are good chemical “fingerprints” for determining the geographic provenance of economic tsavorite from Kenya, Tanzania and Madagascar.
Chemical Geology, 2003
The Crown emerald veins are somewhat enigmatic, displaying characteristics that are common to eme... more The Crown emerald veins are somewhat enigmatic, displaying characteristics that are common to emerald deposits of tectonic -hydrothermal origin and of igneous origin. The veins cut the Fire Lake mafic meta-volcanic rocks, occurring within 600 m of an outcrop of Cretaceous S-type granite. Field work and vein petrography are consistent with a polythermal origin for the veins. The primary vein mineralogy is quartz and tourmaline with variable sized alteration haloes consisting of tourmaline, quartz, muscovite, chlorite and emerald. The veins weather a buff brown colour due to jarosite, scheelite and minor lepidocrocite, which were precipitated during the waning stages of vein formation. Microthermometic studies of primary fluid inclusions within emerald growth zones are consistent with emerald precipitation from H 2 O -CO 2 -CH 4 ( F N 2 F H 2 S) bearing saline brines. The estimated fluid composition is approximately 0.9391 mol% H 2 O, 0.0473 mol% CO 2 , 0.0077 mol% CH 4 and 0.0059 mol% NaCl ( f 2 wt.% NaCl eq.). Fluid inclusion and stable isotope studies are consistent with vein formation in the temperature range 365 -498 jC, with corresponding pressures along fluid inclusion isochore paths ranging from 700 to 2250 bars. These data correlate with a very slow uplift rate for the region of 0.02 -0.07 mm/year. Emerald deposits are generally formed when geological conditions bring together Cr ( F V) and Be. Cr and V are presumed to have been derived locally from the mafic and ultramafic rocks during hydrothermal alteration. The Be is most likely derived from the nearby Cretaceous granite intrusion. D
Marble-hosted ruby deposits represent the most important source of colored gemstones from Central... more Marble-hosted ruby deposits represent the most important source of colored gemstones from Central and South East Asia. These deposits are located in the Himalayan mountain belt which developed during Tertiary collision of the Indian plate northward into the Eurasian plate. They are spatially related to granitoid intrusions and are contained in platform carbonates series that underwent high-grade metamorphism. All occurrences are located close to major tectonic features formed during Himalayan orogenesis, directly in suture zones in the Himalayas, or in shear zones that guided extrusion of the Indochina block after the collision in South East Asia. Ar-Ar dating of micas syngenetic with ruby and U-Pb dating of zircon included in ruby gives evidence that these deposits formed during Himalayan orogenesis, and the ages document the extensional tectonics that were active, from Afghanistan to Vietnam, between the Oligocene and the Pliocene. The petrography shows that ruby-bearing marbles formed in the amphibolite facies (T = 610 to 790°C and P~6 kbar). A fluid inclusion study defines the conditions of gem ruby formation during the retrograde metamorphic path (620 b T b 670°C and 2.6 b P b 3.3 kbar) for the deposits of Jegdalek, Hunza and northern Vietnam. Whole rock analyses of non-ruby-bearing marbles indicate that they contain enough aluminum and chromiferous elements to produce all the ruby crystals that they contain. In addition, (C, O)-isotopic analyses of carbonates from the marbles lead to the conclusion that the marbles acted as a metamorphic closed fluid system that were not infiltrated by externally-derived fluids. The carbon isotopic composition of graphite in marbles reveals that it is of organic origin and that it exchanged C-isotopes with the carbonates during metamorphism. Moreover, the O-isotopic composition of ruby was buffered by metamorphic CO 2 released during devolatilisation of marble and the H-isotopic composition of mica is consistent with a metamorphic origin for water in equilibrium with the micas. The (C, O, H)-isotopic compositions of minerals associated with marble-hosted ruby are all in agreement with the hypothesis, drawn from the unusual chemistry of CO 2-H 2 S-COS-S 8-AlO(OH)-bearing fluids contained in fluid inclusions, that gem ruby formed at P~3 kbar and 620 b T b 670°C, during thermal reduction of evaporite by organic matter, at high temperature-medium pressure metamorphism of platform carbonates during the Tertiary India-Asia collision. The carbonates were enriched in Al-and chromiferous-bearing detrital minerals, such as clay minerals that were deposited on the platform with the carbonates, and in organic matter. Ruby formed during the retrograde metamorphic path, mainly by destabilization of muscovite or spinel. The metamorphic fluid system was rich in CO 2 released from devolatilisation of carbonates, and in fluorine, chlorine and boron released by molten salts (NaCl, KCl,
Alluvial and colluvial gem sapphires are common in the basaltic fields of the French Massif Centr... more Alluvial and colluvial gem sapphires are common in the basaltic fields of the French Massif Central (FMC) but sapphire-bearing xenoliths are very rare, found only in the Menet trachytic cone in Cantal. The O-isotope composition of the sapphires ranges between 4.4 and 13.9‰. Two distinct groups have been defined: the first with a restricted isotopic range between 4.4 and 6.8‰ (n=22; mean δ 18 O=5.6±0.7‰), falls within the worldwide range defined for blue-green-yellow sapphires related to basaltic gem fields (3.0<δ 18 O<8.2‰, n=150), and overlaps the ranges defined for magmatic sapphires in syenite (4.4 < δ 18 O<8.3‰, n = 29). A second group, with an isotopic range between 7.6 and 13.9‰ (n=9), suggests a metamorphic sapphire source such as biotite schist in gneisses or skarns. The δ 18 O values of 4.4-4.5‰ for the blue sapphire-bearing anorthoclasite xenolith from Menet is lower than the δ 18 O values obtained for anorthoclase (7.7-7.9‰), but suggest that these sapphires were derived from an igneous reservoir in the subcontinental spinel lherzolitic mantle of the FMC. The presence of inclusions of columbite-group minerals, pyrochlore, Nb-bearing rutile, and thorite in these sapphires provides an additional argument for a magmatic origin. In the FMC lithospheric mantle, felsic melts crystallized to form anorthoclasites, the most evolved peraluminous variant of the alkaline basaltic melt. The O-isotopic compositions of the first group suggests that these sapphires crystallized from felsic magmas under upper mantle conditions. The second group of isotopic values, typified for example by the Le Bras sapphire with a δ 18 O of 13.9‰, indicates that metamorphic sapphires from granulites were transported to the surface by basaltic magma.
International Geology Review, 1997
The fluid composition, δD of channel H2O, and δ18O of lattice oxygen have been determined in bery... more The fluid composition, δD of channel H2O, and δ18O of lattice oxygen have been determined in beryl and emerald from a variety of geological environments and used to constrain the origin of the parental fluids from which beryl has grown. Step-heating analyses performed by quadrupolar mass spectrometry were used to quantify the composition of the fluid phases in beryl from
Ore Geology Reviews, 2008
Marble-hosted ruby deposits represent the most important source of colored gemstones from Central... more Marble-hosted ruby deposits represent the most important source of colored gemstones from Central and South East Asia. These deposits are located in the Himalayan mountain belt which developed during Tertiary collision of the Indian plate northward into the Eurasian plate. They are spatially related to granitoid intrusions and are contained in platform carbonates series that underwent high-grade metamorphism. All occurrences are located close to major tectonic features formed during Himalayan orogenesis, directly in suture zones in the Himalayas, or in shear zones that guided extrusion of the Indochina block after the collision in South East Asia. Ar-Ar dating of micas syngenetic with ruby and U-Pb dating of zircon included in ruby gives evidence that these deposits formed during Himalayan orogenesis, and the ages document the extensional tectonics that were active, from Afghanistan to Vietnam, between the Oligocene and the Pliocene. The petrography shows that ruby-bearing marbles formed in the amphibolite facies (T = 610 to 790°C and P~6 kbar). A fluid inclusion study defines the conditions of gem ruby formation during the retrograde metamorphic path (620 b T b 670°C and 2.6 b P b 3.3 kbar) for the deposits of Jegdalek, Hunza and northern Vietnam. Whole rock analyses of non-ruby-bearing marbles indicate that they contain enough aluminum and chromiferous elements to produce all the ruby crystals that they contain. In addition, (C, O)-isotopic analyses of carbonates from the marbles lead to the conclusion that the marbles acted as a metamorphic closed fluid system that were not infiltrated by externally-derived fluids. The carbon isotopic composition of graphite in marbles reveals that it is of organic origin and that it exchanged C-isotopes with the carbonates during metamorphism. Moreover, the O-isotopic composition of ruby was buffered by metamorphic CO 2 released during devolatilisation of marble and the H-isotopic composition of mica is consistent with a metamorphic origin for water in equilibrium with the micas. The (C, O, H)-isotopic compositions of minerals associated with marble-hosted ruby are all in agreement with the hypothesis, drawn from the unusual chemistry of CO 2-H 2 S-COS-S 8-AlO(OH)-bearing fluids contained in fluid inclusions, that gem ruby formed at P~3 kbar and 620 b T b 670°C, during thermal reduction of evaporite by organic matter, at high temperature-medium pressure metamorphism of platform carbonates during the Tertiary India-Asia collision. The carbonates were enriched in Al-and chromiferous-bearing detrital minerals, such as clay minerals that were deposited on the platform with the carbonates, and in organic matter. Ruby formed during the retrograde metamorphic path, mainly by destabilization of muscovite or spinel. The metamorphic fluid system was rich in CO 2 released from devolatilisation of carbonates, and in fluorine, chlorine and boron released by molten salts (NaCl, KCl,
Mineralogical Magazine, 2005
Mineralium Deposita, 2011
The combination of oxygen isotope composition with VCr -Mn trace element concentrations of V-bear... more The combination of oxygen isotope composition with VCr -Mn trace element concentrations of V-bearing garnets (tsavorites) originating from the main deposits of the Neoproterozoic Mozambique Metamorphic Belt is reported for the first time. The database enables the identification of the geological and geographical sources of the main productive areas from northern and southern Tanzania, Kenya, and Madagascar. Three consistent sets of δ 18 O values between 9.5‰ and 11.0‰, 11.6‰ and 14.5‰, and 15.5‰ and 21.1‰ have been recognized for primary deposits hosted in graphitic gneisses related to the Neoproterozic metasedimentary series. The δ 18 O value of tsavorite is a good tracer of the environment of its formation; the δ 18 O of the fluid in equilibrium with tsavorite was buffered by the host rock during metamorphism and fluid-rock interaction. This study is a first step in characterizing the geochemistry of gem tsavorite from most of the deposits and occurrences worldwide.
Mineralium Deposita, 2000
Emerald mineralisation in Colombia is located in two distinct zones along the borders of the East... more Emerald mineralisation in Colombia is located in two distinct zones along the borders of the Eastern Cordillera, some 80 km apart. Mineralisation in the western zone has been dated at ca. 35 Ma whereas in the eastern zone it is 30 Ma older. Crush leach analysis of the electrolyte chemistry of¯uid inclusions contained in emerald, quartz, calcite, dolomite and¯uorite from both zones, demonstrates that in each region brines associated with emerald mineralisation range between two extremes with many samples yielding intermediate compositions. Fluid 1, found mainly in emerald-hosted uid inclusions, is dominated by NaCl with high Cl:Br ratios indicating that the salinity was derived by dissolution of halite, most probably from the local salt beds. Fluid 2, found notably in quartz hosted-¯uid inclusions, is of similar salinity but contains less Na and signi®cant concentrations of Ca±K±Fe±Cl and other cations. It has lower Cl:Br ratios, more comparable with formation waters, but is inferred to have obtained part of its salinity by halite dissolution. Bivariate plots of almost all cations have linear or sublinear trends regardless of the mineral hosting the inclusions or the locality from which the samples originated, demonstrating that mixing of the two saline¯uids has occurred. Because the same twō uids occur in both eastern and western zones, despite the dierence in space and time, it is inferred that¯uid compositions were rock controlled by similar interactions with evaporites and black shales in both instances. It is proposed that beryllium was transported as Be±F complexes in the NaCl-¯uid, and was precipitated as emerald after mixing with the calcic brine caused precipitation of¯uorite and parisite.
Mineralium Deposita, 2009
The Ianapera emerald deposit is located in the Neoproterozoic Vohibory Block of southern Madagasc... more The Ianapera emerald deposit is located in the Neoproterozoic Vohibory Block of southern Madagascar. The local geology consists of intercalated migmatitic gneissic units and calcareous metasedimentary rocks, containing boudinaged metamorphosed mafic/ultramafic lenses, all intruded by pegmatite veins. These units occur near the hinge of the tightly folded Ianapera antiform, within a few kilometers of the Ampanihy shear zone. Emerald mineralization is hosted by metasomatic phlogopite veins, and bodies developed within the mafic/ultramafic rocks. Based on field and textural relationships, we distinguish proximal and distal styles of mineralization. Proximal mineralization occurs at the contact of pegmatite veins with mafic/ultramafic units; in the distal style, pegmatites are not observed. Three types of emeralds could be distinguished, mainly on the basis of color and mineral zoning. Some of these emeralds have the most Al-depleted and Cr-rich composition ever recorded. Another characteristic feature to the Ianapera deposit and, to our knowledge, yet unreported, is the association of some emeralds with scapolite in metasomatised mafic rocks. Mineral inclusions are common in most emeralds and include phlogopite, carbonates, barite, K-feldspar, quartz, pyrite, zircon, monazite, bastnaesite, phenakite, plus Fe and Cr oxides. However, feldspar and rare earth element-bearing minerals occur predominantly in proximal emeralds, which also have a more incompatible trace-element signature than distal emeralds. We propose a model related to syn-to post-tectonic magmatic-hydrothermal activity. Pegmatitic bodies intruded units of the Ianapera antiform probably during tectonic relaxation. Exsolution of fluids rich in halogens and incompatible elements from the cooling pegmatites caused hydrothermal metasomatism of Cr-bearing mafic/ultramafic rocks in direct contact with the pegmatites. Local fracturing favored fluid infiltration, permitting the formation of distal mineralization. Emerald composition was controlled by the chemistry of the host rock. The presence of carbonate mineral inclusions in the emeralds and the high F-activity indicated by elevated F-contents in newly formed minerals suggest transport of Be as a fluoridecarbonate complex. It seems likely that beryl formation was triggered by precipitation of F-rich phlogopite, which removed the complexing ligand from the fluid.
Mineralium Deposita, 2008
Alluvial and colluvial gem sapphires are common in the basaltic fields of the French Massif Centr... more Alluvial and colluvial gem sapphires are common in the basaltic fields of the French Massif Central (FMC) but sapphire-bearing xenoliths are very rare, found only in the Menet trachytic cone in Cantal. The O-isotope composition of the sapphires ranges between 4.4 and 13.9‰. Two distinct groups have been defined: the first with a restricted isotopic range between 4.4 and 6.8‰ (n=22; mean δ 18 O=5.6±0.7‰), falls within the worldwide range defined for blue-green-yellow sapphires related to basaltic gem fields (3.0<δ 18 O<8.2‰, n=150), and overlaps the ranges defined for magmatic sapphires in syenite (4.4 < δ 18 O<8.3‰, n = 29). A second group, with an isotopic range between 7.6 and 13.9‰ (n=9), suggests a metamorphic sapphire source such as biotite schist in gneisses or skarns. The δ 18 O values of 4.4-4.5‰ for the blue sapphire-bearing anorthoclasite xenolith from Menet is lower than the δ 18 O values obtained for anorthoclase (7.7-7.9‰), but suggest that these sapphires were derived from an igneous reservoir in the subcontinental spinel lherzolitic mantle of the FMC. The presence of inclusions of columbite-group minerals, pyrochlore, Nb-bearing rutile, and thorite in these sapphires provides an additional argument for a magmatic origin. In the FMC lithospheric mantle, felsic melts crystallized to form anorthoclasites, the most evolved peraluminous variant of the alkaline basaltic melt. The O-isotopic compositions of the first group suggests that these sapphires crystallized from felsic magmas under upper mantle conditions. The second group of isotopic values, typified for example by the Le Bras sapphire with a δ 18 O of 13.9‰, indicates that metamorphic sapphires from granulites were transported to the surface by basaltic magma.
Mineralium Deposita, 2006
The oxygen isotopic composition of gem corundum was measured from 22 deposits and occurrences in ... more The oxygen isotopic composition of gem corundum was measured from 22 deposits and occurrences in Madagascar to provide a gemstone geological identification and characterization. Primary corundum deposits in Madagascar are hosted in magmatic (syenite and alkali basalt) and metamorphic rocks (gneiss, cordieritite, mafic and ultramafic rocks, marble, and calc-silicate rocks). In both domains the circulation of fluids, especially along shear zones for metamorphic deposits, provoked in situ transformation of the corundum host rocks with the formation of metasomatites such as phlogopite, sakenite, and corundumite. Secondary deposits (placers) are the most important economically and are contained in detrital basins and karsts. The oxygen isotopic ratios (18 O/ 16 O) of ruby and sapphire from primary deposits are a good indicator of their geological origin and reveal a wide range of δ 18 O (Vienna Standard Mean Ocean Water) between 1.3 and 15.6‰. Metamorphic rubies are defined by two groups of δ 18 O values in the range of 1.7 to 2.9‰ (cordieritite) and 3.8 to 6.1‰ (amphibolite). "Magmatic" rubies from pyroxenitic xenoliths contained in the alkali basalt of Soamiakatra have δ 18 O values ranging between 1.3 and 4.7‰. Sapphires are classified into two main groups with δ 18 O in the range of 4.7 to 9.0‰ (pyroxenite and feldspathic gneiss) and 10.7 to 15.6‰ (skarn in marble from Andranondambo). The δ 18 O values for gem corundum from secondary deposits have a wide spread between −0.3 and 16.5‰. The ruby and sapphire found in placers linked to alkali basalt environments in the northern and central regions of Madagascar have consistent δ 18 O values between 3.5 and 6.9‰. Ruby from the placers of Vatomandry and Andilamena has δ 18 O values of 5.9‰, and between 0.5 and 4.0‰, respectively. The placers of the Ilakaka area are characterized by a huge variety of colored sapphires and rubies, with δ 18 O values between −0.3 and
Mineralium Deposita, 1998
Oxygen isotopic composition of emerald from 62 occurrences and deposits in the world reveals a wi... more Oxygen isotopic composition of emerald from 62 occurrences and deposits in the world reveals a wide range in d 18 O (SMOW) between +6.2 and +24.7&. The d 18 O-values for each deposit are restricted and can be used to determine the origin of emerald from the world's most important producers. The d 18 Ovalue of emerald appears to be a ®ngerprint of its origin, especially for gems of exceptional quality from Colombia (eastern emerald zone, d 18 O +16.8 0.1&; western emerald zone, d 18 O +21.2 0.5&), Afghanistan (d 18 O +13.5 0.1&), Pakistan (Swat-Mingora districts, d 18 O +15.7 0.1&), Brazil (Santa Terezinha de Goia s, d 18 O +12.2 0.1&; Quadrilatero Ferrifero, d 18 O +6.9 0.4&) and Zimbabwe (Sandawana, d 18 O +7.5 0.5&). Furthermore, the 18 O-composition of emerald appears to be a good marker of its geological environment because the data suggest that host-rock-buering of¯uid d 18 O is considerable during¯uid-rock interaction.
Mineralium Deposita, 1996
A renewal of metallogenetical studies of Colombian emerald deposits produced new geological and g... more A renewal of metallogenetical studies of Colombian emerald deposits produced new geological and geochemical data that favour a hydrothermal-sedimentary genetic model for these deposits. A comprehensive model is presented which integrates both chemical and structural aspects and invalidates some aspects of the model recently presented by Ottaway et al. The deposits result from a two-stage process in which shortening tectonics
In the Guateque-Medina area, Paleozoic basement and Mesozoic rift basins have been uplifted and e... more In the Guateque-Medina area, Paleozoic basement and Mesozoic rift basins have been uplifted and exhumed during the Andean orogeny (12 Ma to present). Surface exposures and subsurface data constrain the deformation style and the rift geometry. We have mapped a regional transect and restored a cross section. We have also reconciled existing stratigraphic data, from cordillera, foothills and foreland basin, and have added new data of our own. In Early Cretaceous shales, there is evidence for fault-controlled sedimentation. A brecciated evaporitic layer, which is locally emerald bearing, has acted as a regional detachment. The underlying basement, composed of Paleozoic sediments, crops out as the Quetame Massif. It was uplifted during the Andean orogeny on a series of high-angle reverse faults. The main SE-verging Tesalia fault has resulted from Andean reactivation of an Early Cretaceous normal fault, which bounded a half-rift. A series of NW-verging back-thrusts may have resulted from Andean reactivation of Paleozoic faults. Between the back-thrusts and the Tesalia fault is a basement pop-up. It may be part of a flower structure, because components of right-lateral slip have been identified. These are attributed to eastward subduction of the Nazca plate beneath South America. In general, the style and timing of Andean deformation in the Guateque-Medina area are compatible with the plate tectonic setting of the northern Andes.
Journal of African Earth Sciences, 2014
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Papers by Gaston Giuliani