Deep Crustal Levels Research Articles

Rapid migration of a magma source from mid- to deep-crustal levels: Insights from restitic granulite enclaves and anatectic granite

Rapid migration of a magma source from mid- to deep-crustal levels: Insights from restitic granulite enclaves and anatectic granite

Petrogenesis of carbonatitic lamproitic dykes from Sidhi gneissic complex, Central India

Petrographic, mineral chemical and whole-rock geochemical characteristics of two newly discovered lamproitic dykes (Dyke 1 and Dyke 2) from the Sidhi Gneissic Complex (SGC), Central India are presented here. Both these dykes have almost similar sequence of mineral-textural patterns indicative of: (1) an early cumulate forming event in a deeper magma chamber where megacrystic/large size phenocrysts of phlogopites have crystallized along with subordinate amount of olivine and clinopyroxene; (2) crystallization at shallow crustal levels promoted fine-grained phlogopite, K-feldspar, calcite and Fe-Ti oxides in the groundmass; (3) dyke emplacement related quench texture (plumose K-feldspar, acicular phlogopites) and finally (4) post emplacement autometasomatism by hydrothermal fluids which percolated as micro-veins and altered the mafic phases. Phlogopite phenocrysts often display resorption textures together with growth zoning indicating that during their crystallization equilibrium at the crystal-melt interface fluctuated multiple times probably due to incremental addition or chaotic dynamic self mixing of the lamproitic magma. Carbonate aggregates as late stage melt segregation are common in both these dykes, however their micro-xenolithic forms suggest that assimilation with a plutonic carbonatite body also played a key role in enhancing the carbonatitic nature of these dykes. Geochemically both dykes are ultrapotassic (K2O/Na2O: 3.0–9.4) with low CaO, Al2O3 and Na2O content and high SiO2 (53.3–55.6 wt.%) and K2O/Al2O3 ratio (0.51–0.89) characterizing them as high-silica lamproites. Inspite of these similarities, many other features indicate that both these dykes have evolved independently from two distinct magmas. In dyke 1, phlogopite composition has evolved towards the minette trend (Al-enrichment) from a differentiated parental magma having low MgO, Ni and Cr content; whereas in dyke 2, phlogopite composition shows an evolutionary affinity towards the lamproite trend (Al-depletion) and crystallized from a more primitive magma having high MgO, Ni and Cr content. Whole-rock trace-elements signatures like enriched LREE, LILE, negative Nb-Ta and positive Pb anomalies; high Rb/Sr, Th/La, Ba/Nb, and low Ba/Rb, Sm/La, Nb/U ratios in both dykes indicate that their parental magmas were sourced from a subduction modified garnet facies mantle containing phlogopite. From various evidences it is proposed that the petrogenesis of studied lamproitic dykes stand out to be an example for the lamproite magma which attained a carbonatitic character and undergone diverse chemical evolution in response to parental melt composition, storage at deep crustal level and autometasomatism.

Contrasting metamorphic evolution of metapelites from the Malpica-Tuy unit and the underlying so-called parautochthon at the coast of NW Spain

Two garnet-chloritoid bearing metapelites from the northern Malpica-Tuy unit (MTU) and a staurolite-andalusite bearing metapelite from the underlying parautochthon were studied to decipher their pressure–temperature (P–T) evolution, mainly based on chemically zoned minerals, such as phengite, using P–T, P–O2, and T–O2 pseudosections. In addition, monazite ages were determined with the electron microprobe in the metapelite from the parautochthon. The chloritoid schists from the MTU experienced peak P–T conditions around 21kbar at 570°C and a subsequent isothermal exhumation. Thus, two high pressure rock types can be distinguished in the MTU: orthogneiss (peak pressures: 13–16kbar) and eclogite+metapelite (peak pressures: 20–23kbar). The latter rock types were exhumed in a subduction channel environment. The staurolite-andalusite bearing schist from the parautochthon experienced only peak pressures around 8kbar at a temperature around 570°C since garnet and potassic white-mica with Si contents above 3.2 per formula unit are lacking. This rock represents sediments that were originally at the surface of the downgoing continental plate and metamorphosed in early Variscan times (around 364Ma) similar to the MTU rocks. Our mean monazite ages of 339 and 320Ma might be related to backthrusting of the MTU rocks from deeper crustal levels onto the parautochthon.

Alteration and dehydration of subducting oceanic crust within subduction zones: implications for d\xe9collement step-down and plate-boundary seismogenesis

The alteration and dehydration of predominantly basaltic subducting oceanic crustal material are thought to be important controls on the mechanical and hydrological properties of the seismogenic plate interface below accretionary prisms. This study focuses on pillow basalts exposed in an ancient accretionary complex within the Shimanto Belt of southwest Japan and provides new quantitative data that provide insight into clay mineral reactions and the associated dehydration of underthrust basalts. Whole-rock and clay-fraction X-ray diffraction analyses indicate that the progressive conversion of saponite to chlorite proceeds under an almost constant bulk-rock mineral assemblage. These clay mineral reactions may persist to deep crustal levels (~320 °C), possibly contributing to the bulk dehydration of the basalt and supplying fluid to plate-boundary fault systems. This dehydration can also cause fluid pressurization at certain horizons within hydrous basalt sequences, eventually leading to fracturing and subsequent underplating of upper basement rock into the overriding accretionary prism. This dehydration-induced breakage of the basalt can explain variations in the thickness of accreted basalt fragments within accretionary prisms as well as the reported geochemical compositions of mineralized veins associated with exposed basalts in onland locations. This fracturing of intact basalt can also nucleate seismic rupturing that would subsequently propagate along seismogenic plate interfaces.Graphical abstractSchematic model showing the setting for décollement step-down into the ocean-plate basalts at the subdution plate interface. The diagram also shows textures of intact basalt and its expected frictional properties and behaviors associated with seismogenesis.

Thermal history and differential exhumation across the Eastern Musgrave Province, South Australia: Insights from low-temperature thermochronology

Multi–method geo- and thermochronological data obtained for Palaeo- and Mesoproterozoic granitoids traversing the main structural architecture of the eastern Musgrave Province within South Australia reveal multiphase cooling histories. Apatite U-Pb dating on six samples yield consistent ages of ~1075–1025Ma, suggesting a thermal reset coinciding with mantle-derived magmatism of the greater Warakurna Large Igneous Province (~1080–1040Ma). Apatite fission track (AFT) analysis indicate that four discrete thermal events affected the study area, inducing cooling through the AFT partial annealing zone (~60–120°C), supported by apatite and zircon (U-Th-Sm)/He data. Late Neoproterozoic cooling from deep crustal levels to temperatures <200°C was discerned, which is thought to be related to exhumation and denudation during the Petermann Orogeny. Subsequent cooling events at ~450–400Ma (Silurian–Devonian) and ~310–290Ma (Late Carboniferous) are interpreted to represent exhumation associated with the Alice Springs Orogeny. The latter event exhumed the sampled plutons to shallow crustal depths. An additional Triassic – early Jurassic thermal event, likely recording elevated geothermal gradients at that time, was observed throughout the study area, however, more data is needed to further support this interpretation. The high sample density across the structural architecture of the study area furthermore reveals patterns of fault reactivation and resulting differential exhumation, indicating shallower exhumation levels in the centre and deeper exhumation towards the margins of the sampled transect. The observed differential exhumation patterns match with existing seismic data and fit a model of an inverted graben system for the Phanerozoic evolution of the eastern Musgraves.The results highlight a complex Phanerozoic thermal history for the eastern Musgraves and help to elucidate the poorly appreciated tectonic evolution of inland Australia. This study further demonstrates how high-density sample transects across structural architecture can assess the relative crustal level and associated preservation of the thermal history record within fault-reactivated terranes.

Precambrian geology of the Kazakh Uplands and Tien Shan: An overview

A comprehensive review of new data on geology and geochronology of Precambrian terranes in the western Central Asian Orogenic Belt reveals new insights into its evolution. At the present surface, these terranes mostly consist of Meso- to Neoproterozoic sedimentary, magmatic and metamorphic assemblages, with insignificant Paleoproterozoic rocks. Archean material is represented exclusively by detrital and xenocrystic zircons in younger strata. Meso- to Neoproterozoic felsic magmatic rocks were mostly sourced from Neoarchean and Paleoproterozoic continental crust, indicating its reworking and potential wider presence at deeper crustal levels. Most Meso- to Neoproterozoic assemblages are of intraplate origin. The supra-subduction assemblages of Neoproterozoic and Mesoproterozoic ages are of limited extent.We propose to recognize the Issedonian and Ulutau-Moyunkum groups of terranes, separated by early Paleozoic Z-shaped ophiolitic suture, based on their different tectono-magmatic evolution in the Mesoproterozoic and Neoproterozoic. Distinctly different are the Mesoproterozoic and early Neoproterozoic assemblages, with lithological variations at the beginning of the late Neoproterozoic and practically no differences at the end of the Neoproterozoic.The Issedonian group of terranes could be part of a Mesoproterozoic (ca. 1100Ma) orogen between the Siberian, North China and Laurentian cratons. The pre-Mesoproterozoic crust of these terranes was completely reworked during the younger events. The Ulutau-Moyunkum group of terranes appear to be lithologically and geochronologically similar to the Tarim craton. Both the Issedonian and Ulutau-Moyunkum groups of terranes were metamorphosed during the Ulutau-Moyunkum event at 700±25Ma.The breakup into currently mappable Precambrian terranes took place during end-Ediacaran to early Paleozoic times after opening of oceanic basins, whose relics are preserved in numerous Paleozoic ophiolitic sutures.

Tectonothermal evolution of a garnet-bearing quartzofeldspathic gneiss from the Moyar shear zone, south India and its bearing on the Neoarchean accretionary tectonics

We present mesoscale structural development across the Nilgiri Block and the flanking Moyar and Bhavani shear zones in south India, and detailed mineral–chemical and geothermobarometric studies of a garnet-bearing quartzofeldspathic gneiss from the easternmost part of the Moyar shear zone. Barring a narrow (<100μm) rim domain, major element distribution within garnet porphyroblasts reveals complete chemical homogenization. The absence of growth zoning in garnet porphyroblasts may suggest a protracted post-garnet growth residence period of the rock at elevated temperatures. Chemical zoning near garnet rim reflects the signature of both retrograde net-transfer (ReNTR) and retrograde exchange (ReER) equilibria. The ReNTR-equilibrium is recognized by prominent Mn kick-up in garnet, whereas the ReER-equilibrium is identified by divergence of Fe and Mg between garnet and biotite. Diffusion modelling, though qualitative, of the observed chemical zoning in garnet suggests an initial phase of rapid (~150°C/Ma) cooling, which may have been achieved by tectonic-extrusion-induced exhumation. Pressure–temperature conditions for peak, ReNTR and ReER are constrained, respectively, at 900°C; 9–11kbar, 735°C; 8kbar and 685°C; 7.8kbar. Analyses of structural fabrics establish oppositely verging nature of the Moyar and Bhavani shear zone and may suggest a doubly vergent orogenic development, with the former as prowedge and the latter as retrowedge. The presence of the Nilgiri Block as a topographically elevated region between these oppositely dipping thrust faults indeed corroborates a doubly vergent orogenic setup. The tectonic scenario is comparable with a continent–continent collision type accretionary tectonics. Peak high-P granulite facies metamorphism and post-peak long residence period of the studied quartzofeldspathic gneiss at deep crustal level suitably fit into the Neoarchean crustal dynamics resulting in crustal thickening, in the order of ~41km, within the Nilgiri Block.

Jurassic cooling ages in Paleozoic to early Mesozoic granitoids of northeastern Patagonia: 40Ar/39Ar, 40K–40Ar mica and U–Pb zircon evidence

U–Pb SHRIMP zircon crystallization ages and Ar–Ar and K–Ar mica cooling ages for basement rocks of the Yaminue and Nahuel Niyeu areas in northeastern Patagonia are presented. Granitoids that cover the time span from Ordovician to Early Triassic constitute the main outcrops of the western sector of the Yaminue block. The southern Yaminue Metaigneous Complex comprises highly deformed Ordovician and Permian granitoids crosscut by undeformed leucogranite dikes (U–Pb SHRIMP zircon age of 254 ± 2 Ma). Mica separates from highly deformed granitoids from the southern sector yielded an Ar–Ar muscovite age of 182 ± 3 Ma and a K–Ar biotite age of 186 ± 2 Ma. Moderately to highly deformed Permian to Early Triassic granitoids made up the northern Yaminue Complex. The Late Permian to Early Triassic (U–Pb SHRIMP zircon age of 252 ± 6 Ma) Cabeza de Vaca Granite of the Yaminue block yielded Jurassic mica K–Ar cooling ages (198 ± 2, 191 ± 1, and 190 ± 2 Ma). At the boundary between the Yaminue and Nahuel Niyeu blocks, K–Ar muscovite ages of 188 ± 3 and 193 ± 5 Ma were calculated for the Flores Granite, whereas the Early Permian Navarrete granodiorite, located in the Nahuel Niyeu block, yielded a K–Ar biotite age of 274 ± 4 Ma. The Jurassic thermal history is not regionally uniform. In the supracrustal exposures of the Nahuel Niyeu block, the Early Permian granitoids of its western sector as well as other Permian plutons and Ordovician leucogranites located further east show no evidence of cooling age reset since mica ages suggest cooling in the wake of crystallization of these intrusive rocks. In contrast, deeper crustal levels are inferred for Permian–Early Triassic granitoids in the Yaminue block since cooling ages for these rocks are of Jurassic age (198–182 Ma). Jurassic resetting is contemporaneous with the massive Lower Jurassic Flores Granite, and the Marifil and Chon Aike volcanic provinces. This intraplate deformational pulse that affected northeastern Patagonia during the Early Jurassic (Sinemurian–Pliensbachian) was responsible for the partial (re)exhumation of the mid-crustal Paleozoic basement along reactivated discrete NE–SW to ENE–WSW lineaments and the resetting of isotopic systems. These new thermochronological data indicate that Early Permian magmatic rocks of the Nahuel Niyeu block were below 300 °C for ca. 20 Ma prior to the onset of the main magmatic episode of the Late Permian to Triassic igneous and metaigneous rocks of the Yaminue block.

Early Paleozoic subduction processes of the Paleo-Asian Ocean: Insights from geochronology and geochemistry of Paleozoic plutons in the Alxa Terrane

The Alxa Terrane is situated in a key area between the North China and Tarim cratons. Paleozoic magmatic records in this terrane place important constraints on the subduction processes of the southern Paleo-Asian Ocean. New data of zircon U-Pb ages and whole-rock elemental and isotopic data reveal two groups of intermediate to felsic plutons in the Alxa Terrane. One group consists of diorites and granitoids that were emplaced at ca. 460–440Ma and characterized by lower Al2O3/TiO2 ratios and higher TiO2 contents, implying high temperature–low pressure crystallization conditions and a shallow source region. The second group is dominated by granitoids aged at ca. 420–407Ma and displays high Sr and Ba, low Y and high rare earth elements, with very high Sr/Y ratios and mostly positive Eu anomalies. These characteristics imply low temperature–high pressure crystallization conditions and source regions at deep crustal levels where garnet is stable in the residual phase. Both of the two groups are mostly calc-alkaline to high-K calc-alkaline, depleted in Nb, Ta and Ti and enriched in Ba, K and Sr, indicative of an arc affinity most likely related to the southward subduction of the Paleo-Asian Ocean. Zircon εHf(t) and whole-rock εNd(t) values of these magmatic rocks decrease from 458Ma to 440Ma and increase from 417Ma to 407Ma, whereas whole-rock initial 87Sr/86Sr ratios display an opposite trend. Such an isotopic change suggests a tectonic switch from an advancing to a retreating subduction regime at ~407Ma. Synthesized data from this and previous studies suggest that the 460–400Ma magmatic arc in the Alxa Terrane represented the western extension of the Paleozoic arc belt on the northern margin of the North China Craton.

Primary Magmas in Continental Arcs and their Differentiated Products: Petrology of a Post-plutonic Dyke Suite in the Tertiary Adamello Batholith (Alps)

Determining the primary compositions of arc magmas is fundamental in retracing the chemical differentiation processes responsible for the formation of juvenile arc crust and the thermal structure of the mantle wedge. We have investigated a series of post-plutonic dykes that intruded the gab- broic to tonalitic southern part of the Tertiary Adamello Batholith in the Alps. The dyke rocks range in composition from primary, hydrous high-Mg basalts to basalts, basaltic andesites, andesites and dacites. Field relationships and high-precision U–Pb dating of titanite and zircon show that the dyke suite ranges in age from 41.67 ± 0.06 Ma for the high-Mg basalt to 38.62 ± 0.12 Ma for the youngest dacitic dykes, closely associated with plutonic activity from 42.5 to 39.0 Ma. Andesites and dacites have primitive 87Sr/86Sri (0.7032–0.7038) and 143Nd/144Ndi (eNdCHUR +3.5–3.2) isotopic signatures strongly limiting the extent of crustal assimilation, whereas some of the high-Mg basalts have se- lectively assimilated pelitic metasedimentary rocks as shown by high Cs/Rb, Rb/Sr and Rb/Zr ratios, and isotopically more enriched compositions (87Sr/86Sri 0.7039–0.7046; eNdCHUR +1.6–0.0). Primitive high-Mg basaltic dykes that escaped assimilation processes are primary mantle partial melts that were extracted from their source at pressures of 2.7 ± 0.2 GPa and temperatures of 1390 ± 30°C, conditions corresponding to the spinel–garnet transition in mantle peridotite. Major elem- ent modelling constrains the degree of melting to 20 6 2% leaving a harzburgite residue, consistent with the trace element chemistry of the high-Mg basalts, which have moderate [Gd/Yb]N ratios of 1–1.2. Differentiated basaltic andesites and dacites follow experimentally constrained liquid lines of descent for fractional crystallization at mid- to deep crustal levels. The trace element chemistry of amphiboles from basaltic andesite and andesite dykes reveals the coexistence of amphibole with primitive melts, indicating elevated pressures and H2O contents in their parental magmas. Thermobarometric constraints for amphibole phenocrysts result in pressures from 0.65 to 0.78 GPa and temperatures ranging from 930 to > 1000°C. The absence of any significant Eu-anomaly in the rare earth element patterns in these amphiboles indicates the late appearance of plagioclase in the crystallization sequence. The crystallization of amphibole drives the differentiated magmas to slightly peraluminous, corundum-normative compositions that are common for tonalites building the major part of the Adamello Batholith. Fractionation models at mid- to lower crustal conditions result in the cumulative crystallization of 17% olivine, 2% Cr-rich spinel, 18% clinopyroxene, 41% amphibole, 4% plagioclase and 0.1% magnetite to obtain an andesitic composition from a primary, hydrous high-Mg continental arc basalt. Cumulates formed during fractional crystallization atmid- to deep crustal levels are dunites and wehrlites followed by hornblendites and hornblende- gabbros. The trace element signatures of basaltic andesites and dacites display low Rb/Zr and Rb/ Sr, and are consistent with fractionation-dominated processes within the crust in an active contin- ental margin. Significant crustal assimilation is not required to obtain the trace element signatures of the evolved andesitic magmas.

Mass transfer and fluid evolution in late-metamorphic veins, Rhenish Massif (Germany): insight from alteration geochemistry and fluid-mineral equilibria modeling

Element mobility and fluid-rock interaction related to the formation of late-metamorphic quartz veins have been studied by combination of mineral chemistry, whole-rock geochemistry, mass balance analysis and fluid-mineral equilibria modeling. The quartz veins are hosted by very low-grade metasedimentary rocks of the fold-and-thrust belt of the Rhenish Massif (Germany). The veins record two stages of evolution, a massive vein filling assemblage with elongate-blocky quartz, chlorite, apatite and albite, and a later open space filling assemblage with euhedral crystals of quartz, ankerite-dolomite and minor calcite and sulfides. Detailed mass balance analysis of an alteration profile adjacent to a representative quartz vein demonstrates that element mobility is restricted to the proximal zone. The most important element changes are gain of Ca, Fe, Mg, Mn, P and CO2, and loss of Si, K and Na. The data demonstrate that wall-rock carbonation is one of the main alteration features, whereas mobility of Si, K and Na are related to dissolution of quartz and destruction of detrital feldspar and muscovite. The whole-rock geochemical data, in conjunction with fluid composition data and pressure-temperature estimates, were used as input for fluid-mineral equilibria modeling in the system Si-Al-Fe-Mg-Ca-Na-K-C-S-O-H-B-F-Cl. Modeling involved calculation of rock-buffered fluid compositions over the temperature interval 100–500 °C, and reaction-path simulations where a rock-buffered high-temperature fluid reacts with fresh host-rocks at temperatures of 400, 300 and 200 °C. Calculated rock-buffered fluid compositions demonstrate that retrograde silica solubility is a strong driving force for quartz leaching in the temperature-pressure window of 380–450 °C and 0.5 kbar. These conditions overlap with the estimated temperatures for the initial stage of vein formation. Reaction-path models show that high-temperature alteration can produce the observed silica leaching, suggesting that fast advection of external hot fluids from deeper crustal levels was essential for the early stage of vein formation. Fluid advection must have occurred as multiple pulses, which allowed for periods of influx of fluids that leached quartz, alternating with periods of cooling and quartz precipitation in the veins. Reaction-path models at high temperatures (300–400 °C) do not produce carbonate alteration, whereas fluid-rock reaction at 200 °C produces carbonate alteration, consistent with the temperature estimates for the late-stage vein carbonate assemblage. Comparison between modeling results and geochemical data suggests that the observed alteration features are the product of fluid-rock reaction under conditions where the external fluid gradually cooled down and evolved with time. The results of this study highlight the importance of late-orogenic fluid migration for the formation of quartz vein arrays in fold-and-thrust belts.

Updated Bouguer anomalies of the Iberian Peninsula: a new perspective to interpret the regional geology

ABSTRACTBouguer anomaly maps are powerful cartographic tools used mainly by geoscientists and natural resources' companies (oil, mining, etc.) since they reflect rock density distribution at different depths, allowing the identification of different tectonic features. At upper crustal levels, Bouguer anomaly maps can help, for instance, in characterizing possible ore deposits, ground water reservoirs, petroleum resources, CO2 storage sites and sedimentary basins; at deeper crustal levels they can help to further refine seismic velocity models or other integrated geophysical models and thus help in deciphering the lateral density variations within the crust and the geometry of the base of the crust. This new Bouguer anomaly map at a 1:1,500,000 scale is based on the compilation of 210,283 gravity stations covering the Iberian Peninsula (c. 583,254 km2). The new map upgrades previous maps in two ways: (1) it is built up from a database with a 15% more spatial coverage than previous compilations and (2) it is freely available. This map show shorter wavelengths than previous published maps thus allowing investigation of smaller geological features.

Petrological Evolution of the Magmatic Suite Associated with the Coroccohuayco Cu(–Au–Fe) Porphyry–Skarn Deposit, Peru

The petrological evolution of magmatic rocks associated with porphyry-related Cu deposits is thought to exert a first-order control on ore genesis. It is therefore critical to understand and recognize petrological processes favourable to the genesis of porphyry systems. In this study we present new petrographic, geochemical (whole-rock and mineral), and isotopic (Pb, Sr, Nd) data for rocks from the magmatic suite associated with the Eocene Coroccohuayco porphyry–skarn deposit, southern Peru. Previously determined radiometric ages on these rocks provide the temporal framework for interpretation of the data. Arc-style magmatic activity started at Coroccohuayco with the emplacement of a composite precursor gabbrodiorite complex at c. 40·4 Ma. After a nearly 5 Myr lull, magmatic activity resumed at c. 35·6 Ma with the rapid emplacement of three dacitic porphyries associated with mineralization. However, zircon antecrysts in the porphyries show that intra-crustal magmatic activity started c. 2 Myr before porphyry emplacement and probably built a large intra-crustal magmatic body with an associated large thermal anomaly. Our data suggest that all magmas underwent a period of evolution in the deep crust before transfer and further evolution in the upper crust. The gabbrodiorite complex was sourced from a heterogeneous deep crustal reservoir and was emplaced at a pressure of 100–250 MPa where it underwent a limited amount of fractionation and formed a chemically zoned pluton. Its initial water content and oxygen fugacity were estimated to be around 3 wt % H2O and NNO ± 1 (where NNO is the nickel–nickel oxide buffer), respectively. The deep crustal source of the porphyries appears to have been more homogeneous. The porphyries are interpreted to be the product of advanced differentiation of a parental magma similar to the gabbrodiorite. Most of this evolution occurred at deep crustal levels (around 800 MPa) through fractionation of amphibole + pyroxene + plagioclase ± garnet, leading to the development of a high Sr/Y signature characteristic of porphyry-related magmatism worldwide. Subsequent upper crustal evolution (100–250 MPa) was dominated by crustal assimilation, cannibalism of previously emplaced magma batches (proto-plutons) and magma recharge. Water content and oxygen fugacity were estimated to be around 5 wt % H2O and NNO + 1 to NNO + 2, respectively, at the end of the period of upper crustal evolution. This high oxygen fugacity is inferred to have favoured sulphur and metal enrichment in the melt. The high thermal regime generated through 2 Myr of sustained magmatism in the upper crust favoured crustal assimilation, proto-pluton cannibalism, and efficient metal extraction upon fluid exsolution. The Coroccohuayco magmatic suite appears to have acquired its metallogenic potential (high fO2, high Sr/Y) through several million years of deep crustal evolution.

Nd isotopic composition of the Paleozoic granitic rocks and structure of the deep crustal levels of the Chingiz Range, East Kazakhstan

This work presents the Nd isotopic composition of Middle Cambrian to Late Paleozoic granitic rocks of the central part of the Chingiz Range in eastern Kazakhstan. All the rocks are characterized by a high positive ɛNd(t) value (no less than +4), which is similar to that of the country island arc volcanic rocks. These peculiarities allow us to suggest that the juvenile crust of this region was formed at the island arc stage. The continental crust was further thickened due to the transformation of the island arc rocks with an insignificant contribution of the crustal component. On the basis of these data, it is assumed that the deep crustal levels of the Chingiz Range are composed of tectonically juxtaposed, geochemically and isotopically similar, metamorphosed complexes of the Early Paleozoic island arcs.

Polyphase deformation and metamorphism of the Cuiabá group in the Poconé region (MT), Paraguay Fold and Thrust Belt: kinematic and tectonic implications

Several deformation models have been proposed for the Paraguay Belt, which primarily differ in the number of phases of deformation, direction of vergence and tectonic style. Structural features presented in this work indicate that the tectonics was dominated by low dip thrust sheets in an initial phase, followed by two progressive deformation phases. The first phase of deformation is characterized by a slate cleavage and axial plane of isoclinal recumbent folds with a NE axial direction, with a recrystallization of the minerals in the greenschist facies associated with horizontal shear zones with a top-to-the-SE sense of movement. The second stage shows vergence towards the NW, characterized by crenulation cleavage axial plane to F2 open folds over S0 and S1, locally associated with reverse faults. The third phase of deformation is characterized by subvertical faults and fractures with a NW direction showing sinistral movement, which are commonly filled by quartz veins. The collection of tectonic structures and metamorphic paragenesis described indicate that the most intense deformation at the deeper crustal level, greenschistfacies, occurred during F1, which accommodated significant crustal shortening through isoclinal recumbent folds and shear zones with low dip angles and hangwall movement to the SE, in a thin-skinned tectonic regime. The F2 deformation phase was less intense and had a brittle to ductile behavior that accommodated a slight shortening through normal open subvertical folds, and reverse faults developed in shallower crustal level, with vergence towards the Amazonian Craton. The third phase was less pervasive, and the shortening was accommodated by relief subvertical sinistral faults.

Magma storage and plumbing of adakite-type post-ophiolite intrusions in the Sabzevar ophiolitic zone, northeast Iran

Abstract. Subduction-related adakite-type intrusive rocks emplaced into the late Cretaceous–Paleocene Sabzevar ophiolite zone, northeast Iran, range from Mg-andesite to rhyodacite in composition. Here we investigate the magma supply system to these subvolcanic intrusive rocks by applying thermobarometric mineral and mineral–melt equilibrium models, including amphibole thermobarometry, plagioclase–melt thermobarometry and clinopyroxene–melt barometry. Based on the results of these thermobarometric models, plagioclase crystallized dominantly at pressures of ~350 (130 to 468) MPa, while amphiboles record both low pressures (~300 MPa) and very high pressures (&gt;700 MPa) of crystallization. The latter is supported by the calculated pressures for clinopyroxene crystallization (550 to 730 MPa). The association of amphibole with clinopyroxene and no plagioclase in the most primitive samples (Mg-andesites) is consistent with amphibole fractionation from very hydrous magmas at deep crustal levels of the plumbing system, which may have been a key process in intensifying adakite-type affinities in this rock suite. Barometry, combined with frequent disequilibrium features such as oscillatory-zoned and sieve-textured plagioclase crystals with An-rich overgrowths in more evolved samples, implies that final magma differentiation occurred in an open upper crustal magma system that developed progressively stronger compositional modifications during high-level magma storage.

Mineralogy, Fluid Characteristics, and Re-Os Age of the Late Triassic Dahu Au-Mo Deposit, Xiaoqinling Region, Central China: Evidence for a Magmatic-Hydrothermal Origin

The Dahu Au-Mo deposit is located in the Xiaoqinling region and belongs to the Qinling-Dabie orogen of central China. The quartz vein-style Dahu Au-Mo deposit (31 t Au, average grade: 4.7 g/t; 30,000 t Mo, average grade: 0.13 %) formed in four stages, i.e., an early quartz-K-feldspar stage (I), a pyrite-molybdenite stage (II), a sulfide-telluride-sulfosalt-gold stage (III), and a late carbonate-barite stage (IV). Abundant tellurides and sulfosalts occur in stage III: altaite, tellurobismuthite, buckhornite, hessite, petzite, calaverite, aikinite, kupcikite, lindstromite, salzburgite, wittichenite, and tetradymite. The system is characterized by carbono-aqueous fluids of low to moderate salinity and high oxygen fugacity. Total homogenization temperatures of the H 2 O-CO 2 fluid inclusions in stage I and stage III quartz range from 230° to 440°C and 198° to 320°C, respectively. The variation in CO 2 /H 2 O ratios and microthermometric data of the H 2 O-CO 2 fluid inclusions results from a combination of fluid unmixing and sequential trapping of fluid inclusions, but also from postentrapment reequilibration. Abundant minerals are present as solid phases in H 2 O-CO 2 fluid inclusions in stage I quartz: Cu 1.65 S, covellite, chalcopyrite, bornite, molybdenite, pyrite, colusite, anhydrite, and celestine. δ 18 O quartz values of stage I quartz range from 10.2 to 12.0‰, with calculated δ 18 O fluid values ranging from 5.6 to 7.4‰. The δ D values of fluid inclusion waters (thermal decrepitation) in stage I quartz range from −117 to −54‰. The low and scattered δ D values reflect a mixture of ore-forming fluids trapped by the H 2 O-CO 2 fluid inclusions at deep crustal levels and postmineralization fluids, possibly meteoric waters, trapped by secondary aqueous fluid inclusions at higher crustal levels during uplift of the Xiaoqinling core complex. Four molybdenite samples yield an Re-Os isochron age of 206.4 ± 3.9 Ma (MSWD = 0.88), suggesting ore formation in the Late Triassic. Metal signature, mineral association, fluid characteristics, and isotope data of the Dahu deposit, together with the tectonomagmatic setting, imply that the ore-forming fluids were derived from an unexposed and oxidized magmatic system, which likely stems from partial melting of enriched lithospheric mantle beneath the North China craton in a postcollisional extensional setting.

Geophysical constraints for terrane boundaries in southern Mongolia

AbstractThe Central Asian Orogenic Belt (CAOB) is a typical accretionary orogen divided into numerous lithostratigraphic terranes. In theory, these terranes should be characterized by contrasting magnetic and gravity signatures owing to their dissimilar petrophysical properties. To test this hypothesis, the extent of tectonostratigraphic terranes in southern Mongolia was compared with the potential field data. The analysis reveals that the terrane boundaries are not systematically defined by strong gravity and magnetic gradients. The correlation of the magnetic signal with the geology reveals that the magnetic highs coincide with Late Carboniferous to Early Permian volcanic‐plutonic belts. The matched filtering shows a good continuity of signal along the boundaries of these high magnetic anomalies toward the deeper crustal levels which may indicate the presence of deeply rooted tectonomagmatic zones. The axes of high‐density bodies in the western and central parts of the study area are characterized by periodic alternations of NW‐SE trending gravity anomalies corresponding to up to 20 km wide cleavage fronts of Permo‐Triassic age. The matched filtering analysis shows good continuity of signal to the depth of these gravity highs which may indicate presence of deeply rooted high‐strain zones. The magnetic signal is interpreted to be as the result of a giant Permo‐Triassic magmatic event associated with lithosphere‐scale deformation, whereas the gravity pattern is related to the postaccretionary shortening of the CAOB between the North China and Siberia cratons.

Coeval Felsic and Mafic Magmas in Neoarchean Calc-alkaline Magmatic Arcs, Dharwar Craton, Southern India: Field and Petrographic Evidence from Mafic to Hybrid Magmatic Enclaves and Synplutonic Mafic Dykes

Abstract We present field and petrographic data on Mafic Magmatic Enclaves (MME), hybrid enclaves and synplutonic mafic dykes in the calc-alkaline granitoid plutons from the Dharwar craton to characterize coeval felsic and mafic magmas including interaction of mafic and felsic magmas. The composite host granitoids comprise of voluminous juvenile intrusive facies and minor anatectic facies. MME, hybrid enclaves and synplutonic mafic dykes are common but more abundant along the marginal zone of individual plutons. Circular to ellipsoidal MME are fine to medium grained with occasional chilled margins and frequently contain small alkali feldspar xenocrysts incorporated from host. Hybrid magmatic enclaves are intermediate in composition showing sharp to diffused contacts with adjoining host. Spectacular synplutonic mafic dykes commonly occur as fragmented dykes with necking and back veining. Similar magmatic textures of mafic rocks and their felsic host together with cuspate contacts, magmatic flow structures, mixing, mingling and hybridization suggest their coeval nature. Petrographic evidences such as disequilibrium assemblages, resorption, quartz ocelli, rapakivi-like texture and poikilitically enclosed alkali feldspar in amphibole and plagioclase suggest interaction, mixing/mingling of mafic and felsic magmas. Combined field and petrographic evidences reveal convection and divergent flow in the host magma chamber following the introduction of mafic magmas. Mixing occurs when mafic magma is introduced into host felsic magma before initiation of crystallization leading to formation of hybrid magma under the influence of convection. On the other hand when mafic magmas inject into host magma containing 30-40% crystals, the viscosities of the two magmas are sufficiently different to permit mixing but permit only mingling. Finally, if the mafic magmas are injected when felsic host was largely crystallized (∼70% or more crystals), they fill early fractures and interact with the last residual liquids locally resulting in fragmented dykes. The latent heat associated with these mafic injections probably cause reversal of crystallization of adjoining host in magma chamber resulting in back veining in synplutonic mafic dykes. Our field data suggest that substantial volume of mafic magmas were injected into host magma chamber during different stages of crystallization. The origin of mafic magmas may be attributed to decompression melting of mantle associated with development of mantle scale fractures as a consequence of crystallization of voluminous felsic magmas in magma chambers at deep crustal levels.

A volcanic province near the western termination of the Charlie-Gibbs Fracture Zone at the rifted margin, offshore northeast Newfoundland

A mid-Cretaceous to Late Cretaceous volcanic province, named here the Charlie-Gibbs Volcanic Province, is described near the western termination of the Charlie-Gibbs Fracture Zone, against the rifted continental margin northeast of Newfoundland. We used seismic data to map 14 volcanic seamounts, now buried below younger sediments. They rise 0.7 to 2 s two-way time (twt) above the surrounding basement level and are about 8–30 km wide. Some are conical while others are more flat-topped. Underlying igneous units resembling flows and sills are also observed. Based on magnetic modeling of the large positive magnetic anomalies associated with the seamounts, the total thickness of igneous rocks can locally reach about 8 km. This magmatism occurred in the vicinity of the Charlie-Gibbs Fracture Zone and extends about 150 km to the north along the rifted continental margin. The volcanic province also forms the northern boundary of the Jurassic-Early Cretaceous Orphan Basin, along a major transform margin there. Truncation of rift-related structures which extend to deep crustal levels is observed at the transform, along trends similar to those of prerift Appalachian terrane boundaries on the adjacent shelf. This suggests the existence of a preexisting weak zone in the continental lithosphere within which a complex strike-slip fault system developed and may have controlled the location of final continental breakup between the Rockall and North American plates in the Late Cretaceous.