Granulite Grade Research Articles

IN-SIMS zircon U-Pb geochronology from the Snowbird tectonic zone Large igneous Province (STZ LIP), western Churchill Province, Canada

IN-SIMS (in-situ secondary ionization mass spectrometry) micro-zircon geochronology resolves the age of the Chipman and Kazan mafic dike swarms exposed along the central segment of the Snowbird Tectonic Zone (STZ), the geophysically defined feature separating the Rae and Hearne subprovinces of the western Churchill Province. Long-considered syn-kinematic with respect to Paleoproterozoic transpressional deformation, the primitive composition of the STZ Large Igneous Province (LIP) and granulite grade metamorphic overprint has made establishing crystallization ages difficult. In-situ U-Pb isotopic analysis of micro-zircon yielded scattered and discordant results, except for four zircon crystals, which yielded a weighted mean of 2113 Ma +/- 22 (n = 8 analyses; MSWD: 0.46). This age is similar to other mafic complexes in the western Churchill Province including the Griffin Gabbro sills interpreted to have formed during opening of the Manikewan Ocean. Nd isotopic analyses of Kazan dikes paired with geochemical data suggest that the Chipman and Kazan dikes were asthenospherically-derived but contaminated by a metasomatized lithospheric root. At ca. 2.1 Ga, the STZ was the site of intracratonic extension within a coherent Rae-Hearne crustal block (Snowbird LIP Phase I). Existing data suggests that subsequent tectonism led to focused dextral transpression along the trace of the dike swarm and attests to a long-lived interplay of preexisting weaknesses and deformation.

A continental back-arc setting for the Namaqua belt: Evidence from the Kakamas Domain

A study of the NW Kakamas Domain in South Africa/Namibia provides a new, unified lithostratigraphy and evolutionary history applicable to the whole Namaqua Sector. The Mesoproterozoic history ranges from ∼1350 Ma to 960 Ma, but isotopic evidence suggests it was built upon pre-existing Paleoproterozoic continental crust that extended west from the Archaean Craton. In eastern Namaqualand, early rift-related magmatism and sedimentation at ∼1350 Ma occurred in a confined ocean basin. Subsequent tectonic reversal and subduction at ∼1290–1240 Ma led to establishment of the Areachap, Konkiep and Kaaien Domains. In the Kakamas Domain, widespread deposition of pelitic sediments occurred at ~1220 Ma (Narries Group). These contain detrital zircons derived from proximal crust with ages between ∼2020 Ma and 1800 Ma (western Palaeoproterozoic domains) and 1350–1240 Ma (eastern early Namaqua domains), suggesting pre-sedimentation juxtaposition. The pelites underwent granulite grade metamorphism at ∼1210 Ma (peak conditions: 4.5–6 kbar and 770–850 °C), associated with voluminous, predominantly S-type granitoid orthogneisses between ∼1210 Ma and 1190 Ma (Eendoorn and Ham River Suites) and low-angle ductile (D2) deformation which continued until ∼1110 Ma, interspersed with periods of sedimentation. This enduring P-T regime is inconsistent with the expected crustal over-thickening associated with the generally-accepted collision-accretion Namaqualand model. Rather, we propose the Namaqua Sector is a ‘hot orogen’ developed in a wide continental back-arc with subduction west of the present-day outcrop. The observed high geotherm resulted from thinned back-arc lithosphere accompanied by an influx of mantle-derived melts. Ductile D2 deformation resulted from “bottom-driven” tectonics and viscous drag within the crust by convective flow in the underlying asthenospheric mantle. This extended tectonothermal regime ceased at ∼1110 Ma when SW-directed thrusting stacked the Namaqua Domains into their current positions, constrained in the Kakamas Domain by late- to post-tectonic I-type granitoids intruded between ∼1125 Ma and 1100 Ma (Komsberg Suite). The thermal peak then shifted west to the Bushmanland and Aus Domains, where voluminous granites (1080–1025 Ma) were associated with high-T/low-P granulite facies thermal metamorphism and mega-scale open folding (D3). Unroofing of the Namaqua Sector is marked by large-scale, NW-trending, sub-vertical transcurrent dextral shear zones and associated pegmatites and leucogranites at ∼990 Ma.

Zircon as a Recorder of Trace Element Changes during High-Grade Metamorphism of Neoarchean Lower Crust, Shevaroy Block, Eastern Dharwar Craton, India

AbstractSystematic changes in whole-rock chemistry, mineralogy, mineral textures, and mineral chemistry are seen along a ca. 95-km traverse of late Archean granitoid orthogneisses in the Shevaroy Block, Eastern Dharwar Craton, southern India. The traverse passes from amphibolite-grade gneisses in the north to granulite-grade rocks (charnockite) in the south. Changes include whole-rock depletion of Rb, Cs, Th, and U in the granulite grade rocks as relative to the amphibolite grade gneisses, and oxidation trends regionally from highly oxidised granulite-facies rocks near the magnetite–haematite buffer to relatively reduced amphibolite-facies rocks below the fayalite-magnetite-quartz. Rare earth elements show limited mobility and are hosted a variety of minerals whose presence is dependent on the metamorphic grade ranging from titanite and allanite in the amphibolite-facies rocks to monazite in the vicinity of the orthopyroxene-in isograd to apatite in the granulite-grade charnockite. Cathodoluminescence and back-scattered electron sub-grain imaging and sensitive high-resolution ion microprobe analysis of zircon from 29 samples of dioritic, tonalitic, and granitic orthogneiss from the traverse reveals magmatic zircon cores that record the emplacement of the granitoid protoliths mostly about 2580 to 2550 Ma, along with a few older mid to late Archean tonalites. Protolith zircon was modified during metamorphism by overgrowth and/or replacement. Relative to igneous cores, U-enriched metamorphic zircon, dominant in the amphibolite-grade gneisses, formed at ca. 2530 Ma, predating retrograde titanite growth at ca. 2500 Ma. Uranium-depleted mantles grew on zircon between 2530 and 2500 Ma in granulite-grade samples south of the orthopyroxene-in isograd. In some of these samples, the U-depleted metamorphic zircon is preceded by mantles of U-undepleted zircon, indicating a progression of metamorphic zircon growth with increasingly depleted compositions between 2530 and 2500 Ma. With increasing metamorphic grade (from amphibolite to granulite) and oxidation state, allanite and monazite disappear from the assemblage and zircon became depleted in U and Th. Whole-rock U-Th compositions became decoupled from relict magmatic zircon compositions, reflecting the development of U-depleted metamorphic zircon and indicating that whole-rock chemical differences along the traverse were produced during metamorphism, rather than just reflecting differences in dioritic vs granitic protoliths. Although in situ anatexis and melt extraction may have played a role, whole-rock and zircon depletion of trace elements can be explained by the action of externally derived, oxidising, low-H2O activity hypersaline fluids migrating up through the mid to lower crust. Fluids and element migration during metamorphism may be the end result of subduction related processes that cumulated in the collision and concatenation of island arcs and continental blocks. These tectonic processes assembled the Dharwar Craton at the end of the Archean.

Mesoarchaean (2820 Ma) high-pressure mafic granulite at Uauá, São Francisco Craton, Brazil, and its potential significance for the assembly of Archaean supercratons

High pressure (HP) granulites of regional scale form as a result of tectonic events that lead to crustal thickening or subduction of the crust into the mantle. Most HP granulites are Phanerozoic, a few are Proterozoic, and Archaean HP granulites are even scarcer. Here we present field relationships, mineral chemistry and zircon U-Pb ages, Hf isotope data and trace elements data for the mafic granulite and associated rocks of the Uauá terrane, São Francisco Craton, Brazil, as evidence for the likely existence of a thick continental crust in the Mesoarchaean/Neoarchaean transition. The HP mafic granulite occurs as lensoid bodies within shallow dipping diorite to leucodiorite gneisses. Small scale layering between mafic granulite and diorite gneiss indicate these rocks are cogenetic. Garnet-clinopyroxene pairs with quartz, zircon, ilmenite, plagioclase, and clinopyroxene inclusions in garnet characterize the HP assemblage. Garnet porphyroblasts also show opx-cpx-plag symplectite coronas, which coupled with hornblende and plagioclase define PT conditions to lower grade granulite and amphibolite facies. Microprobe data combined with phase equilibria modelling (pseudosections) indicate 16–18 kbar and 930–960 °C for the peak HP assemblage, and 6.2–7.0 kbar and 660–760 °C for lower pressure granulite to amphibolite facies symplectite coronas. Metamorphic zircon rims in equilibrium with garnet have SHRIMP ages of 2819 ± 14, and igneous zircon cores of 3127 ± 14 Ma. The cores of zircon in associated gneiss samples have U-Pb ages between 3090 ± 13 Ma (LA-ICP-MS) and 3125 ± 15 Ma (SHRIMP) with age cluster at 3120 ± 6 Ma. εHf(t) values on igneous zircon of the HP mafic granulite are slightly positive whereas metamorphic zircon rims are negative. The associated diorite gneiss invariably yielded negative zircon εHf(t) values. Trondhjemite sheets intrusive in the mafic granulite are ca. 20 m.y. younger (2794 ± 13 Ma) than the host granulite. We interpret the igneous protoliths of the mafic granulite and leucodiorite gneiss as a single igneous complex emplaced in the continental crust, later deformed and metamorphosed by contraction and crustal thickening during collision of blocks/cratons to form Earth's first supercratons by the end of the Mesoarchaean.

Monazite and xenotime U–Th–Pb $$_{\mathrm{total}}$$ total ages from basement rocks of the (central) Shillong–Meghalaya Gneissic Complex, Northeast India

Monazite and xenotime are the two most useful and commonly used geochronometers for deciphering ages from metamorphic rocks. The low analytical cost involved in electron probe micro-analyser chemical dating, ease of sample preparation and abundance in metamorphic rocks of wide P–T conditions make monazite and xenotime dating most widely used technique for age determination amongst metamorphic petrologists. This contribution presents age comparisons between coexisting monazite and xenotime in the basement metapelitic rocks of the central part of the Shillong–Meghalaya Gneissic Complex (SMGC). Thermobarometric estimates in the studied samples indicate granulite facies conditions of metamorphism with peak \({P{-}T}\) conditions of \(\sim \)6.5 kbar and \({\sim }750^\circ \hbox {C}\). Results indicate that xenotime in the basement rocks in the central SMGC formed in four discrete geological events while monazite either formed only in the latest Pan-African granulite grade metamorphic event or recrystallised during this event. Monazite in the studied samples yielded a single ubiquitous age of ca. 500 Ma. Xenotime in the study area, although found in only one sample, preserves four distinct ages at \(1153 \pm 29,930 \pm 36,823 \pm 41\; {\hbox {and}}\; 490 \pm 11\;\hbox {Ma}\). Preservation of Grenvillian ages in xenotime from central SMGC marks the eastward extension of Rodinia amalgamation front in the Indian Shield. The Neoproterozoic ages in xenotime from central SMGC suggest that the ca. 820 Ma high-grade metamorphism in the Eastern Indian Tectonic Zone had a wider impact in the SMGC than perceived previously.

The Grenvillian Namaqua fold belt adjacent to the western Kaapvaal Craton: 2. Archaean Craton and supercontinent connections

The Namaqua sector of the Namaqua-Natal Province consists of large tectonostratigraphic terranes, some of which includes Paleoproterozoic fragments inherited from the supercontinent Columbia. Most of the terranes became part of Rodinia during the Mesoproterozoic Namaqua Orogeny (1.3–1.0Ga, Grenvillian). The terranes in the western foreland of the Kaapvaal Craton (Kheis Subprovince) form a thin-skin fold and thrust belt (5–15km depth to sole thrust). Westerly directed thrusting towards the hinterland is reminiscent of thick-skin tectonics, associated with the exhumation of high grade facies terranes, such as the granulitic and migmatitic Grünau Terrane. Upper amphibolite Upington Terrane was thrusted from the west onto thrust sheets contiguous to Kaapvaal Craton, in turn over-ridden near Prieska by the Grünau Terrane. Severity of the ductile deformation is illustrated by regional shear zones, ubiquitous recumbent isoclinal folds and allochthonous mega sheath folds in the Grünau Terrane. Vergence reversal of fold and thrust structures along a narrow (10–20km) northwesterly trending zone (Vergence Inversion Zone, VIZ), represents a crustal scale pop-up structure coincident with the Namaqua Front, a paired low-high gravity anomaly linked to a mega-scale anticline-syncline pair caused by deep-seated thrusting of the Columbian Moho.Late Columbian extension (1.6–1.3Ga, diachronous with early Rodinian events) produced parautochthonous basins with volcano-sedimentary sequences, deformed during the Namaqua Orogeny. Early to mid-Columbian (2.3–1.9Ga) rocks form basement to the mobile belt, with Steinkopf Terrane as type outcrop area (probably time-equivalent with the Grünau and Pofadder Terranes, Sperrgebiet Domain, Rehoboth structural Province and part of the Congo Craton). The western boundary of the Kaapvaal Craton (and composite Kalahari Craton) exhibits structural wedge edges of Archaean Kaapvaal basement and Ventersdorp-Vaalian basins. Columbia-Kaapvaal Detachment (COLKAD), a décollement between Kaapvaal and Columbia cratons (footwall) and Olifantshoek and other Namaqua terranes (hanging wall) extends for >600km westwards.Namaqua crust was thickened (20–25km) by both structural stacking and sheeted intrusions (Namaqua tectogenesis) with clockwise PTt-paths in the west-central Namaqua sector (Olifantshoek, Grünau, Pofadder, Bladgrond Terranes). In the western part (Okiep/Garies Terrane), crust was dominantly thickened by massive sheetlike granitoid intrusions resulting in granulite grade and anti-clockwise PTt-paths. COLKAD-rooted inter/intra-terrane thrusts are subhorizontal except where rotated by subvertical (discrete) shear zones (e.g. Tantalite Valley and Brakbos-Doringberg-Dabep). Crustal thickening and granulite-uplift stimulated temperature rise with anatexis-palingenesis of early Columbian/supracrustal rocks. Kilometer-scale granitoid bodies, derived from predominantly Columbian crust (according to isotopic source rock models) that are emplaced along shear zones, caused a rise in the brittle to ductile transition zone (i.e. mid-crustal conditions) and low-pressure granulite facies.

Magnetic Anomaly Interpretation of the Northern Congo Craton Boundary: Results from Depth Estimation and 2.5D Modeling

A magnetic-based geophysical study was performed across the southern part of Cameroon to investigate the boundary between the Archean Congo craton and the Pan-African metamorphic belt. Magnetic gradient techniques including Euler deconvolution and Tilt derivative have been applied to an aeromagnetic data profile to determine the depth of sources and their lateral extension. 2.5D magnetic modeling shows that the prominent magnetic positive anomalies observed on total magnetic map of south Cameroon are produced by deep and strongly magnetic bodies under the Pan-African formations mainly an important dyke formation structure with a high susceptibility of 0.041 (SI units), at an average depth of 4148 m and with a lateral extension of about 10 km. These bodies are interpreted to have emplaced at high crustal levels in a continental collision zone and were subsequently metamorphosed at granulite grade conditions, during the Pan-African orogeny about 620 Ma ago.

The Pelona–Orocopia–Rand and related schists of southern California: a review of the best-known archive of shallow subduction on the planet

ABSTRACTThe Pelona–Orocopia–Rand and related schists of southern California are an archetypal example of an exhumed shallow subduction complex. ‘The schist’ comprises mainly trench materials underthust beneath continental arc rocks during Late Cretaceous–early Cenozoic collision of one or more oceanic plateaux with southern California. The arc-on-trench relationship, without intervening mantle or lowermost crust, implies that significant subduction erosion accompanied shallow subduction. Upsection increases in metamorphic grade (~150 ± 100°C/km) and spatial variations in age and peak temperature provide an ~50 million year long record of tectonic underplating within a cooling system. Evidence for palaeoseismic events in earliest formed and hottest (locally transitional granulite grade) schists provides a possible field-based record of episodic tremor and slow slip events such as detected in several modern shallow subduction zones. Structural ascent of the schist was achieved in distinct Late Cretaceous–early Eocene and late Oligocene–early Miocene extensional pulses, the first during collapse of gravitationally unstable upper plate assemblages and accompanied by trench-directed (top-NE) lower plate extrusion and the second corresponding temporally, spatially, and in character with core complex formation in the SW United States. The line between schist and core complex belts is blurred by the recent discovery of schist within 40 km of the nearest core complex and containing synkinematic Miocene intrusions, a hallmark of SW U.S. core complexes. The history of schist assembly, metamorphism, and exhumation provides the most complete field-based record of thermo-mechanical processes, subduction erosion and tectonic underplating in particular, that operated during a shallow subduction event. Future cross-disciplinary investigations of, and comparisons between, the schist and other possible ancient (e.g. Swakane gneiss, Sanbagawa belt, Qiangtang terrane) and modern (e.g. Cascadia, SW Japan, central Mexico, Chile) shallow subduction zones will yield new insights into the tectonic and petrologic processes that operate within such systems.

Geochronological constraints on the Hartbees River Thrust and Augrabies Nappe: New insights into the assembly of the Mesoproterozoic Namaqua-Natal Province of Southern Africa

The region around Augrabies in the Namaqua sector provides insights into the processes that formed the Namaqua-Natal tectonic Province. Progressive ductile shear deformation during the long lasting Namaqua Orogeny was coeval with the global Grenvillian Orogeny. The Mesoproterozoic Namaqua-Natal Province is composed of severely deformed and metamorphosed rocks in terranes bounded by major thrust and shear zones. The interpreted research results deal with events around the Hartbees River Thrust, a suture zone where the granulite grade Grünau (Kakamas) terrane was overthrust onto the amphibolite grade Bladgrond (Bushmanland) terrane. The series of distinct deformation stages D1–D6 that have been identified during previous research in the western regions of the Namaqua sector, is also discernible in the study region around Augrabies. Microbeam geochronological techniques were used to date zircons from four samples of a granitic rock (the Karama’am Augen Gneiss) intrusive into the Hartbees River Thrust zone, and of a sample of granite (the Augrabies Granite) from the core of a large allochthonous sheath fold nappe in the Grünau terrane. The mean crystallisation age of the Karama’am intrusive is 1108±4Ma and of the Augrabies Granite 1168±6Ma. The sheath fold core of the latter granite is enclosed in the sheetlike Rooipad Granite (Riemvasmaak Granite) previously determined to be 1155±7Ma. Crustal residence times deduced from Lu–Hf isotopic compositions of zircon cores and xenocrysts, cluster around 1710Ma for the Augrabies Granite and about 200Ma older for the Karama’am Granite Gneiss (with the oldest date at 2069Ma). Collectively the available data enable reconstruction of the sequence of Namaqua Orogeny deformation stages: terrane assembly (D1) at ∼1195Ma with metamorphism at 1191±12Ma, pervasive folding and thrusting (D2) at 1168±6Ma, formation of major sheath and coaxial folds (D3) at 1155±7Ma, late open folds (D4) at 1090±16 and a late metamorphism (coeval with major shear zones D5–D6?) at 1018±11 to 1024±14Ma.

Tsumoite (BiTe) and associated Ni-PGE mineralization from Gondpipri mafic-ultramafic complex, Bastar Craton, Central India: mineralogy and genetic significance

Abstract This paper reports the occurrence of Tsumoite (a bismuth telluride) in the Heti Cu-Ni-PGM prospect, Gondpipri mafic-ultramafic complex, Central India. The Gondpipri complex consists of several tectonically dismembered gabbronorite-gabbro-anorthositic gabbro — olivine gabbro -websterite disposed in ∼10 km long tonalite-trondhjemitegranodiorite (TTG) and charnockite-enderbite suite of rocks. The mineralization occurs in the sulphide zone hosted by gabbro variants. The host rocks have been deformed and metamorphosed to granulite grade and subjected to various degrees of hydrothermal alteration. The mineralization comprises chalcopyrite, pentlandite, pyrrhotite, cubanite, millerite, and pyrite. In addition to these, occur (1) tsumoite (2) PGM in the form of moncheite, merenskyite, Pd-mellonite, and Pt-Pd-Te-Bi-Fe-S alloy. The present study indicates that the mineralization occurs in two stages related to: (i) magmatic and (ii) hydrothermal remobilization and transport of Cu-rich sulphides, tsumoite and PGM, and their re-deposition in hydrosilicate alteration zones. It is possible that the mineralization at Heti formed at different stages of bismuth activity under variable fS2, T, and fTe2 conditions due to change in total concentration of Te and S and /or cooling. Since the role of S is limited, Te and cooling are important factor influencing mineralogy and composition of tsumoite and associated mineralization. Mineralization occurs in two different modes of occurrences. The early mineralisation occur as blebs, specks and dissemination of sulphides, viz. pyrrhotite, chalcopyrite, pentlandite and minor pyrite ± PGM, whereas later mineralisation occur as stringers, minor veins of sulphides viz. pyrite, millerite, cubanite, sijenite, tsumoite and ± PGM. Mineral assemblages and textural relationships at Heti has indicated precipitation of tsumoite and associated PGM along fractures and secondary silicates, which confirms their hydrothermal origin.

Comparison of the metamorphic history of the Monapo Complex, northern Mozambique and Balchenfjella and Austhameren areas, Sør Rondane, Antarctica: Implications for the Kuunga Orogeny and the amalgamation of N and S. Gondwana

Reconstructions of Gondwana place Dronning Maud Land, Antarctica (DML) adjacent to N. Mozambique prior to fragmentation. The Monapo Complex outlier klippen overlying the Nampula Terrane in N. Mozambique has been correlated with rocks in eastern and central DML. Metamorphic assemblages and P–T conditions from the two areas are compared as well the timing of metamorphism. Granulite grade assemblages preserved in the Monapo Complex suggest P–T conditions of ∼900°C and >10kb. Textures vary with reactions typical of isothermal decompression, isobaric cooling and hydration being recognized but also include equilibrium assemblages. P–T estimates from four samples suggest initial inversion of high pressure assemblages from at least ∼10kb and ∼900°C to mid-crustal levels where near isobaric cooling and hydration at between ∼4–7kb and 550–700°C is recognized. Granulite grade assemblages from Balchenfjella in eastern Sør Rondane, DML suggest initial P–T conditions of ∼900°C and >10kb. Textures vary with reactions typical of decompression, cooling and hydration being recognized but also include equilibrium assemblages. P–T estimates from four samples suggest initial inversion of high pressure assemblages from at least ∼10kb and ∼900°C to mid-crustal levels where retrogression at between ∼6–7kb and 600–700°C is recognized.SHRIMP zircon data from six samples from eastern Sør Rondane show episodic zircon growth over a period from ∼630Ma to ∼530Ma with five metamorphic pulses being defined. Undeformed granitic intrusions with ages of ∼550Ma intrude Sør Rondane. SHRIMP zircon data from samples from alkaline intrusions and mafic granulites in the Monapo Complex yield crystallization and metamorphic ages of ∼635Ma and 570-590Ma respectively. The complex is intruded by granitic veins with ages of ∼550Ma.The similarities in metamorphic mineral assemblages, P–T estimates and zircon geochronology data indicate a common geological history between Sør Rondane and the Monapo Complex of northern Mozambique. This common history is interpreted to result from both areas being part of a mega-nappe structure which was emplaced in a transpressional setting involving collision between N. Gondwana (comprising rocks of the East African Orogeny) and south Gondwana (comprising the Nampula Terrane of northern Mozambique and the Maud Province of western Dronning Maud Land) along the Damara-Zambesi-Lurio-CDML collision boundary.Other erosional remnants of this nappe include the Mugeba Complex in northern Mozambique, the Kataragama klippe in Sri Lanka, the Urungwe klippe in Zimbabwe, the Masosa Suite and Mavhuradohna Complex in western Mozambique and NE Zimbabwe.

Petrology of the Neoproterozoic granulites from Central Dronning Maud Land, East Antarctica – Implications for southward extension of East African Orogen (EAO)

Suturing of east and west Gondwana occurred during the Neoproterozoic along the East African Orogen (EAO) through continent–continent collision. The southern extension of EAO is projected in central Dronning Maud Land (CDML) in east Antarctica. CDML is considered to be a magmato-metamorphic terrain where high-grade metamorphism occurred during Mesoproterozoic. Granulite grade metamorphic assemblages from metapelites (garnet+sillimanite+K feldspar (perthite)+graphite±plagioclase+quartz±melt), mafic granulite (orthopyroxene+garnet±clinopyroxene+plagioclase+quartz) and granitic orthogneiss (garnet+Fe-clinopyroxene+K feldspar+plagioclase+quartz) in the Humboldt Mountains in CDML have been used to deduce the peak metamorphic conditions as ∼850°C and 6–8kbar pressure. In situ chemical dating of monazite in matrix and inclusions in garnet indicates metamorphism to have occurred between 640 and 580Ma and was partially overprinted at ∼540Ma which is correlatable with extensive charnockite and A-type granite emplacements. This Neoproterozoic metamorphism and its correlation with the coast margin Neoproterozoic granulites at Schirmacher oasis is inferred as the extension of EAO in east Antarctica.

Sphalerite from the Sterling Hill Mine, New Jersey: Origin and History

The Sterling Hill and Franklin zinc-iron-manganese deposits, Sussex County, New Jersey, are sedimentary-exhalative in origin but were highly deformed and metamorphosed to granulite grade during the Mesoproterozoic Ottawan orogeny. They are unique because the major zinc minerals, zincite, franklinite, and willemite, are oxides and silicates rather than sulfides. However, at Sterling Hill minor granular sphalerite occurs in the central black willemite zone associated with loelling and graphite and sparsely elsewhere in the deposit. Based on textural observations and stable isotope geochemistry, sphalerite was present during metamorphism and was most likely part of the protolith mineral assemblage. It likely formed from a fluid containing seawater sulfate that circulated through volcanic rocks and reacted with igneous sulfides; some contribution from biological reduction of sulfate seems likely. This interpretation agrees with the generally accepted model that the protolith formed on the floor of an ancient rift in an environment similar to the present-day Red Sea brine pool sediments. The presence of loellingite and absence of pyrite or arsenopyrite in the black willemite zone requires that fAs2−/fS2− >1 during metamorphism. The position of the black willemite zone, occupying most of the central cross member of the deposit, and the more reduced mineral assemblages of the zone relative to the rest of the deposit are compatible with it being the feeder zone for the stratiform ores that make up the rest of the Sterling Hill deposit, indicating that the deposit is inverted relative to its original position. Pressure-corrected temperatures from fluid inclusion geothermometry and temperatures determined from carbon isotope fractionation factors between coexisting graphite and calcite suggest a postmetamorphic event that occurred at ∼360°–280°C at between ∼820 and 750 Ma. The fluid inclusion and mineral assemblage data also suggest that at least one additional postmetamorphic event affected the sphalerite and loellingite and their coexisting minerals. This event involved the development of secondary fluid inclusions in sphalerite and the formation of a secondary manganese arsenosilicate mineral and may correlate with the pervasive serpentinization-like alteration of the deposit. The complex sequence of vents indicated by the textures, isotopic ratios, and fluid inclusions is compatible with the history of prolonged cooling and uplift experienced by the New Jersey Highlands in the Neoproterozoic.

Constraining cooling histories: rutile and titanite chronology and diffusion modelling in NW Bhutan

AbstractU–Pb analyses of rutile and titanite commonly yield ages that constrain the timing of cooling rather than the timing of their crystallization. Rutile which grew at or close to peak temperature conditions in a mafic granulite, intermediate granulite and mafic amphibolite within juxtaposed litho/tectonostratigraphic units in the Greater Himalayan Sequence (GHS) of NW Bhutan yield LA–MC–ICP–MS U–Pb lower intercept cooling ages of 10.1 ± 0.4, 10.8 ± 0.1 and 10.0 ± 0.3 Ma, respectively. Numerical finite‐difference diffusion models constrained by previously published temperature–time and Pb diffusion data suggest that these ages are best explained by rapid cooling from peak temperature conditions of ∼800 °C at 14 Ma in the granulite‐bearing unit and ∼650 °C at 12 Ma in the amphibolite‐bearing unit. The good fit between the model and analysed ages confirms the relatively high retention of Pb in rutile suggested by the experimental data. Titanite that grew during an exhumation‐related amphibolite facies overprint on an eclogite facies mineral assemblage from the neighbouring Jomolhari Massif yields a U–Pb lower intercept cooling age of 14.6 ± 1.2 Ma. Diffusion modelling suggests that this age is too old to be consistent with the temperature–time paths inferred for the rutile‐bearing samples. Instead, the titanite age suggests cooling from ∼650 °C at an earlier time of 17–15 Ma, implying that the high‐grade rocks in the Jomolhari Massif experienced a different cooling history from the rest of the GHS in NW Bhutan. Together these data show that high‐grade rocks from three apparently different structural levels of the GHS in NW Bhutan experienced rapid cooling at >40 °C Ma−1 at varying times. The highest grade granulite facies rocks were exhumed from deeper structural levels that are not exposed, not preserved, or not yet recognized west of eastern Nepal. A progressive along‐strike change in tectonic regime, metamorphic history and/or exhumation mechanism across the orogen is implied by these thermochronologic data.

A Unified Model of Neoarchean-Proterozoic Convergence and Rifting of Indian Cratons: Geophysical Constraints

Neoarchean and Proterozoic sutures and collision zones are identified in the Indian Peninsular Shield based on high seismic velocity; gravity highs and high conductivity in the upper crust due to thrusting while sub- ducted side are demarcated based on geophysical signatures of crustal thickening and back arc type basins. Some of them appear to form triple junctions. The Bouguer anomaly map of the south Indian shield when transformed to apparent density map through harmonic inversion, provided high density linear zones coin- ciding with the shear zone and the transition zone-the Moyar Bhavani Shear Zone (MBSZ) between the Eastern Dharwar Craton (EDC) and the Western Dharwar Craton (WDC) and the Dharwar cratons and the Southern Granulite Terrain (SGT), respectively. It is supported by high seismic velocity and high conductiv- ity suggesting them to be caused by high grade granulite rocks related to Neoarchean-Paleoproterozoic su- tures and collision zones. These investigations also suggest thick crust (~40 - 50 km) under the WDC and the SGT forming crustal root of 50 - 52 km in the south western part and thin crust of 31 - 32 km under the EDC indicating direction of convergence and subduction as E-W and N-S between the EDC and the WDC and Dharwar cratons and the SGT, respectively. It gave rise to contemporary lower crustal granulite rocks in the northern part of the SGT and Cauvery shear zone (CSZ) as collision related central core complex of various deep seated intrusive rocks of Paleo-Mesoproterozoic period. The second case belonging to Meso-proterozoic period is related to the collision of the Bundelkhand craton and the Bhandara-Bastar craton (BBC) and the Dharwar craton (DC) in Central India along the Satpura Mobile Belt (SMB) and the BBC and the DC along the Godavari Proterozoic Belt due to N-S and NE-SW convergences, respectively. This process has given rise to lower crustal granulite rocks of high density, high velocity and high conductivity along the SMB and the GPB. An upper mantle conductor delineated south of the western part of the SMB under Dec- can Volcanic Province and a regional gravity gradient almost sub parallel to it indicate an interface with flu- ids separating rocks of different densities that appears to demarcate the trace of the Proterozoic subduction and suture related to the SMB collision zone during Mesoproterozoic period. High reflectivity of the lower crust along seismic profiles across the SMB indicate an extensional phase prior to this convergence. The SMB is connected to the Aravalli Delhi Mobile Belt (ADMB) in the western part that is another collision zone of Meso-proterozoic period, forming an arcuate shaped collision zone between the Bundelkhand craton and Rajasthan block with E-W convergence. There are indications of a prior phase of convergence during Paleo-Proterozoic period followed by rifting during Paleo-Meso-proterozoic period (~1.9 - 1.6 Ga) along the SMB, the ADMB and the GPB that gave rise to large scale contemporary intrusive in these sections. The contemporary Mahakoshal-Bijawar and Pakhal group of rocks of Paleo Proterozoic period (~1.9 - 1.6 Ga) were deposited over the rifted platform of the Bundelkhand craton along the SMB and cratons along the GPB, respectively during the extensional phase as suggested above based on high reflectivity of the lower crust. It is followed by deposition of the Vindhyan sediments of Meso-Neoproterozoic period (~1.6 - 0.7 Ga) along the SMB and the ADMB as foreland basins during Meso-Neoproterozoic convergence. Simultaneous N-S and E-W directed convergences in the two cases, viz., the SMB and the ADMB that are connected forming an arcuate shaped collision zone suggest NE-SW directed primary stress direction similar to the GPB that is supported by NW-SE oriented large lineaments in Bundelkhand craton and Peninsular shield. The Eastern Ghat Mobile Belt (EGMB) also shows signatures of E-W or NE-SW directed Mesoproterozoic (~1.5 - 1.0 Ga) convergence with East Antarctica. This convergence was preceded by Paleo-Mesoproterozoic rifting (~1.9 - 1.6 Ga) that gave rise to contemporary activities of the EGMB and large scale volcanic activity that formed several basins west of it.

Lithogeochemistry as a tracer of the tectonic setting, lateral integrity and mineralization of a highly metamorphosed Mesoproterozoic volcanic arc sequence on the eastern margin of the Namaqua Province, South Africa

The Areachap Group represents the medium- to high-grade metamorphic and deformed remnants of a Mesoproterozoic (ca. 1.24–1.30 Ga) volcanic arc that was accreted onto the western margin of the Kaapvaal Craton during the early stages of the Namaqua-Natal Orogeny. Few regional geochemical studies of the succession have been undertaken and despite recent geochronological work from throughout the outcrop belt there is still uncertainty as to the lateral integrity and correlation of the succession from one area to another. This uncertainty has been aided by minimal outcrop and extensive deformation and dislocation of one area from another. This study uses trace element geochemistry combined with Sm–Nd-isotopic data from the northern and southern ends of the succession to test its lateral integrity. Minor differences, including the abundance of certain lithologies, varying influences of a crustal component and differences in age between different areas are apparent, but the succession, as a whole, displays lateral integrity as a continental island arc. All lithologies are characterised by enrichment in Th, U, Pb and the large ion lithophile elements relative to primitive mantle, along with Nb–Ta depletion relative to La, and strong light rare earth element (LREE) enrichment relative to heavy REE depletion, which, along with positive ε Nd(t) values of mafic gneisses from both ends of the succession, indicates mantle-derivation within a subduction-related environment with a substantial amount of crustal assimilation. Despite some minor geochemical and isotopic differences in the lithological architecture on a local scale the Areachap Group exhibits coherent geochemical characteristics along its entire strike length. Localised remobilization of elements, particularly in areas characterised by granulite grade metamorphism, as well as in the altered footwall zones of volcanogenic massive sulphide lenses, is present. These alteration trends, along with primary geochemical signatures are remarkably well preserved despite the high grade of metamorphism, as illustrated in clearly defined hydrothermal alteration trends on the alteration box plot, and serves as an efficient tool to delineate areas that have undergone hydrothermal alteration spatially associated with massive sulphide ore bodies. The systematic association of hydrothermal alteration and sulphide ore bodies provides support for the isochemical character of rock compositions during high-grade metamorphism and the lack of extensive post-metamorphic alteration processes.

Cumberland batholith, Trans-Hudson Orogen, Canada: Petrogenesis and implications for Paleoproterozoic crustal and orogenic processes

Large volume, plutonic belts, such as the ∼ 221,000 km 2, ca. 1.865–1.845 Ga Cumberland batholith (CB) of the Trans-Hudson Orogen in Canada, are major components of Paleoproterozoic orogenic belts. In many cases, they have been interpreted as continental arc batholiths. The petrogenesis and tectonic context of the CB and implications for crustal growth and recycling are interpreted herein based on a 900 km geochemical-isotopic (Nd–O) transect across it and into granitoid plutons within bounding Archean cratons in central and southern Baffin Island. The mainly granulite grade CB, emplaced over an age span of between 14 and 24 Ma, consists mainly of high-K to shoshonitic monzogranite and granodiorite, but also includes low- and medium-K granitoid rocks. Metaluminous to slightly peraluminous compositions and δ 18O (VSMOW) values (+ 6 to + 10‰) indicate derivation from infracrustal (I-type) sources. ε Nd 1.85 Ga signatures (− 12 to − 2) of both mafic and felsic units suggest a dominance of evolved sources. Isotopic signatures in the interior of the CB (− 2 to − 7) are more radiogenic than those within Archean domains in central (− 8 to − 15) and southern (− 5 to − 19) Baffin Island. The isotopic transect is interpreted as ‘imaging’ an accreted microcontinental block (Meta Incognita) and bounding Archean cratons. The CB includes granites of arc, within-plate (A-type) and post-collisional affinity and volumetrically minor mafic rocks with both arc and non-arc features. (La/Yb) CN and Sr/Y values range from < 1 to 225 and < 1 to 611, respectively. In these respects, some CB granitoid rocks resemble Paleozoic adakitic granites, interpreted as partial melts of greatly thickened crust within post-collisional settings, such as Tibet. Thus, the CB likely encompasses various non-consanguineous magmatic suites generated at deep- to mid-crustal depths. Although CB granitoid rocks undoubtedly had important crustal sources, it is hard to assess the relative contribution of mantle-derived magmas. The CB is best interpreted as a post-accretion batholith resulting from large-scale lithospheric mantle delamination followed by the upwelling of hot asthenospheric mantle leading to voluminous crustal partial melting. Contributors to crustal instability which may have facilitated such delamination included: (a) a collage of recently assembled small cratons underlain by hot, weak lithosphere with mantle-depth structural breaks within this segment of the Trans-Hudson Orogen; (b) the gabbro-eclogite phase transformation, and (c) a greatly thickened crustal section (> 60 km), as evidenced by adakitic granites.

Fingerprinting a multistage metamorphic fluid–rock history: Evidence from grain scale Sr, O and C isotopic and trace element variations in high-grade marbles from East Antarctica

Granulite grade marble layers interlayered with metapelitic granulites from Lützow Holm Bay, East Antarctica, provide insight into fluid–rock interactions during burial to and exhumation from lower crustal levels. Sub-millimeter scale strontium, oxygen and carbon isotope variations along with LA-ICPMS trace element geochemistry and mineral chemistry of texturally characterized carbonates and associated minerals helped to reconstruct the multistage metamorphic fluid history. Fluid–rock interaction dating back to prograde metamorphism are still preserved in consistently low oxygen and high strontium isotope compositions ( δ 18O = 12‰; 87Sr/ 86Sr (550Ma) = 0.7248) within a massif dolomitic marble layer that escaped significant later metasomatism. In most marbles, total re-crystallization and isotopic resetting occurred in the presence of “externally derived” hyper-saline fluids that circulated along the carbonate layers during the early stages of prograde metamorphism. This leads to a trend of increased radiogenic Sr in marbles towards the value of associated metapelitic rocks that have 87Sr/ 86Sr (550Ma) of 0.764. LA-ICPMS studies on trace elements in carbonate and associated silicate minerals at different textural settings, distinguished using cathodoluminescence microscopy, revealed multiple metasomatic events during retrograde metamorphism. Trace element contents of Ba, Sr, Pb and U gave compelling evidence for metasomatic alteration that postdate the exsolution of carbonate at ~ 600 ºC, which can be correlated with the fluids released from the crystallization of anatectic melts and pegmatites. Subsequently, meteoric fluid infiltration occurred at a shallower level of the crust and caused extreme oxygen isotopic heterogeneity ( δ 18O = 14.7 ~ − 4.9‰) and imprinted high concentration of fluid mobile elements. Taken together our results emphasize the importance of integrating textural and chemical heterogeneities to reveal the multiple episodes of fluid–rock interaction processes in a dynamic continental crust, which has major implications on migration of fluids and material and help in formulating models on the geodynamic evolution of crust.

Weathering of lower crustal rocks in the Kaveri river catchment, southern India: Implications to sediment geochemistry

In the upper catchment of the Kaveri river in the Sahyadri mountains of southern India, middle to lower crustal Archean granulite grade mafic and felsic rocks with similar structures and textures are exposed under the conditions of active tectonics, high rainfall and thick tropical vegetation. Occurrence of the two major rock types in close association under identical geological, geographical and biological conditions provides an uncommon situation for the study of weathering, elemental mobilization and sediment generation processes. Field observations, mineralogical and geochemical data including major, trace and rare earth elements (REE) of fresh rocks and variably weathered saprolite samples suggest that close association of mafic and felsic rocks accelerates the denudational processes by early weathering of mafic minerals in felsic rocks and mafic rocks in the terrain. Due to differential weathering of rocks, unweathered to less weathered felsic grains are likely transferred to the coarser fraction of fluvial sediments deposited on the floodplains of the river imposing an upper continental crust (UCC) geochemical signature. It is found that during chemical weathering, in addition to other factors, weatherability of host minerals of REE control the mobility of REE in the weathering profile. It is suggested from the observations on the weathering process and on the geochemistry of derivative sediments, that in a tectonically active system with a climate maximum, as in Sahyadris, an equilibrium could be dynamically maintained between weathering and erosional regimes. Also we infer from our study that there exist certain commonalities between surface denudational and mantle-magmatic geochemical differentiation processes. Similarity of these processes, therefore, may have implication to common UCC-like geochemistry of Post Archean sediments.

Metamorphism, graphite crystallinity, and sulfide anatexis of the Rampura–Agucha massive sulfide deposit, northwestern India

Located adjacent to the Banded Gneissic Complex, Rampura–Agucha is the only sulfide ore deposit discovered to date within the Precambrian basement gneisses of Rajasthan. The massive Zn–(Pb) sulfide orebody occurs within graphite–biotite–sillimanite schist along with garnet–biotite–sillimanite gneiss, calc–silicate gneisses, amphibolites, and garnet-bearing leucosomes. Plagioclase–hornblende thermometry in amphibolites yielded a peak metamorphic temperature of 720–780°C, whereas temperatures obtained from Fe–Mg exchange between garnet and biotite (580–610°C) in the pelites correspond to postpeak resetting. Thermodynamic considerations of pertinent silicate equilibria, coupled with sphalerite geobarometry, furnished part of a clockwise P–T–t path with peak P–T of ∼6.2 kbar and 780°C, attained during granulite grade metamorphism of the major Zn-rich stratiform sedimentary exhalative deposits orebody and its host rocks. Arsenopyrite composition in the metamorphosed ore yielded a temperature [and log f(S 2)] range of 352°C (−8.2) to 490°C (−4.64), thus indicating its retrograde nature. Contrary to earlier research on the retrogressed nature of graphite, Raman spectroscopic studies on graphite in the metamorphosed ore reveal variable degree of preservation of prograde graphite crystals (490 ± 43°C with a maximum at 593°C). The main orebody is mineralogically simple (sphalerite, pyrite, pyrrhotite, arsenopyrite, galena), deformed and metamorphosed while the Pb–Ag-rich sulfosalt-bearing veins and pods that are irregularly distributed within the hanging wall calc–silicate gneisses show no evidence of deformation and metamorphism. The sulfosalt minerals identified include freibergite, boulangerite, pyrargyrite, stephanite, diaphorite, Mn–jamesonite, Cu-free meneghinite, and semseyite; the last three are reported from Agucha for the first time. Stability relations of Cu-free meneghinite and semseyite in the Pb–Ag-rich ores constrain temperatures at >550°C and <300°C, respectively. Features such as (1) low galena–sphalerite interfacial angles, (2) presence of multiphase sulfide–sulfosalt inclusions, (3) microcracks filled with galena (±pyrargyrite) without any hydrothermal alteration, and (4) high contents of Zn, Ag (and Sb) in galena, indicate partial melting in the PbS–Fe0.96S–ZnS–(1% Ag2S ± CuFeS2) system, which was critical for metamorphic remobilization of the Rampura–Agucha deposit.