Peak Metamorphism Research Articles

Age, petrogenesis, and metamorphic modelling of high‐pressure garnet‐amphibolite from the Tethyan Himalayan Sequence of Bhagirathi Valley, Western Garhwal Himalaya

The Cenozoic Himalayan Orogeny is a manifestation of subduction and subsequent collision of the Indian lithosphere with the Asian Plate in the Eocene, resulting in the formation of intra‐terrane shear zones and regional metamorphic belt along the strike of the entire orogen. This metamorphic belt contains high‐pressure (HP) metamorphic rocks (garnet‐amphibolite, late‐stage granulite, and host gneisses) at various structural levels in the Himalayas. In the present study, we carried out geochemical, chronological, and metamorphic modelling of an exotic tschermakite‐bearing garnet‐amphibolite unit exposed within the Tethyan Himalayan Sequence of Western Garhwal Himalaya of India. Bulk‐rock geochemistry suggests the protolith's tholeiitic fractionation trend and trachy‐andesitic nature. U–Pb geochronology of zircon reveals an age spectrum with pronounced clusters at ~1,550 Ma, ~1,350 Ma, 1,100–1,150 Ma, 900–930 Ma, and ~550 Ma. There is a possibility that the protolith of the HP garnet amphibolite is magmatic. However, near‐concordant ages at certain intervals might suggest crustal components from multiple sources. Integrated geothermobarometry and P–T pseudosection modelling indicate high‐P/medium‐T conditions (~1.65 GPa and 650°C) of peak metamorphism. Based on these results, we infer that this garnet‐amphibolite represents pre‐Himalayan Indian lower crust and during India–Asia collision. The leading edge of the Indian continental lithosphere subducted where mafic and some of the felsic lithologies underwent HP garnet‐amphibolite facies metamorphism, and after a possible slab break‐off, these rocks exhumed during the Himalayan orogeny.

Late Mesoproterozoic to early Neoproterozoic evolution of the Meghalaya gneissic complex, NE India: significance in Rodinia assembly

ABSTRACT The Meghalaya Gneissic Complex (MGC) of northeast India is a little-studied high-grade metamorphic terrain, located close to the Late Palaeoproterozoic to early Neoproterozoic orogenic front. We include mapping, petrography, mineral chemistry, geochemistry, phase equilibria modelling, and U-Th-total Pb monazite geochronology to investigate the evolutionary history of deformed and metamorphosed ortho- and para-gneisses from the central MGC. The metapelite and metamafic rocks of the MGC are folded to E-W upright, antiforms-synforms; and intruded by charnockites during progressive deformations. Reconstructing the metamorphic history of pelite (garnet-cordierite granulite) and two pyroxene granulite, using a TiMnNCKFMASH and TiNCKFMASHO system, reveals a clockwise P–T path where the peak metamorphism was associated with mid crustal burial heating to granulite facies (~825°C and 5.5 to 6.5 kbar) and subsequent cooling with a minor decompression (~ 816°C and 5.3 kbar). Chemically zoned monazites were grown episodically during 1035 ± 13 Ma (n = 21), 976 ± 14 Ma (n = 16), and 494 ± 14 Ma (n = 26). The granulite metamorphism here initiated at 1035 ± 13 Ma and reached thermal peak at 976 ± 14 Ma. This contribution emphasizes the importance of late Mesoproterozoic to early Neoproterozoic metamorphic evolution of the MGC related to the collision of North and South Indian cratonic blocks. The studied P-T path differs from the previously reported Meso- to early Neoproterozoic paths of Central and Western India and close to Western Australia. This finding further explains the change in the early Neoproterozoic collisional pattern of the Central Indian Suture in extreme east (near MGC) and its significance in reconstructing the western Rodinia.

Role of Hydrothermal Circulation along and above Inherited Basement Structures Relating to Unconformity-Related Uranium Mineralization

The reactivation of inherited tectonic structures formed during the Paleoproterozoic Trans-Hudson Orogeny (THO) has played a significant role in generating high-grade unconformity-related uranium deposits in the eastern Athabasca Basin. The role of these tectonic structures is now investigated through a series of two-dimensional hydrothermal numerical models. Two modelling scenarios are considered: (1) models during the THO peak of metamorphism and (2) models with a permeable layer mimicking the presence of the Athabasca Basin, deposited unconformably over the THO basement. In the first scenario, general fluid patterns are strongly affected by the applied permeability configurations. Unidirectional high fluid flow zones (from 10-9 to 10-8 m·s-1) and high thermal gradients (up to 65°C·km-1) can be observed above and within the deep-seated tectonic structures. In the second scenario, well-established fluid convection cells or unidirectional fluid flow zones are observed within the basin layer, with upflow originating from the core of the deep-seated structures, regardless of the applied fluid pressure regime. These results highlight that these deep-seated structures can efficiently transport fluids and heat towards the upper parts of the crust and the basin. In the second scenario, the loci for preore alteration are then evaluated by computing a rock alteration index based on temperature and fluid velocity constraints. These alteration areas reside along and above the deep-seated structures and are potential regions for structural reactivation during mineralization. These results imply that the analysis of the inherited tectonic structures, combined with the alteration regions, can serve as markers for uranium exploration.

Petrochronological evolution of Mg-Al granulites and associated metapelites from the contact zone of the Archean Bastar Craton and Proterozoic Eastern Ghats Province, and its implications

• Mg-Al granulites and khondalites in the Bastar Craton-Eastern Ghats Belt interface. • Ultrahigh-temperature peak metamorphic conditions along a clockwise P-T path. • Mg-Al granulites record decompression-cooling from 8.5kbar, 925°C to 5.5kbar, 770°C. • Texturally controlled in-situ monazite dating reveal two distinct age populations. • Tonian age amalgamation of the Bastar craton with the EGP and subsequent reworking. Mg-Al granulites and associated metapelites occurring in a shear zone at the contact of the Archean Bastar Craton and the Proterozoic Eastern Ghats Province (EGP) have been studied here. Reaction textures, geothermobarometry and P-T pseudosection studies reveal that the rocks record ultrahigh peak temperatures (>900°C) at pressures of 8-8.5 kbar and were exhumed along a retrograde decompression-cooling trajectory reaching ∼770°C and 5-6 kbar following an overall clockwise P-T path. Texturally constrained in-situ monazite chemical dates establish the age of peak metamorphism at 920 ± 11 Ma. Another prominent age cluster is at ∼500 Ma, which is ascribed, on the basis of petrographic characteristics, to tectonothermal reworking in the presence of reactive fluid. UHT metamorphism at ∼920 Ma along a clockwise path and extensive amphibolite facies reworking accompanying fluid infiltration at ∼500 Ma for the studied rocks contrast with the known petrological history of the EGP. We tentatively correlate these features in the contact shear zone to Tonian age amalgamation of the Bastar Craton with the EGP and subsequent tectonothermal reworking, aided by fluid infiltration, during the Kuunga orogeny.

IN SITU U-Pb MONAZITE GEOCHRONOLOGY RECORDS MULTIPLE EVENTS AT THE MOUNT ISA Cu (± Zn-Pb-Ag) DEPOSIT, NORTHERN AUSTRALIA

Abstract The Mount Isa Cu (± Zn-Pb-Ag) deposit is the largest Cu deposit in the Western fold belt of the Mount Isa inlier. Previous geochronological studies on the deposit have produced a large range (>150 m.y.) in ages for Cu mineralization and associated hydrothermal alteration. This study combines detailed petrology with in situ monazite U-Pb geochronology on four monazite-bearing samples in order to constrain the age of hydrothermal and tectonic events experienced by the Mount Isa Cu (± Zn-Pb-Ag) deposit and enclosing host shale. Samples EY108402 and EX102476 contain singular subangular monazite grains included in dolomite and siderite, which are associated with premineralization silica-dolomite alteration. Monazite from these samples yields mean weighted 207Pb/206Pb ages of 1587 ± 43 (mean square of weighted deviates [MSWD] = 0.57) and 1623 ± 25 Ma (MSWD = 0.61), respectively. These ages constrain the maximum age of silica-dolomite alteration and Cu mineralization, reflecting monazite growth during periods of peak metamorphism and early basin inversion, respectively. A sample from the 1100 Cu orebody (DDR012-2) contains two clusters of fine-grained monazite that replace siderite associated with silica-dolomite alteration, envelop chalcopyrite, and are crosscut by chlorite-quartz-orthoclase microveins. Monazite from these clusters produces 207Pb/206Pb ages ranging from ca. 1620 to ca. 1360 Ma. The large variation in ages is attributed to variable radiogenic Pb loss from a precursor monazite due to (1) continuous coupled dissolution-reprecipitation reactions over ca. 260 m.y. or (2) partial recrystallization by a ca. 1360 Ma fluid event. As monazite from this sample envelops chalcopyrite, the ca. 1360 Ma age can be used to infer the minimum age of Cu mineralization. Sample 1758-1 is from a highly silicified and fractured section of the Eastern Creek Volcanics located adjacent the deposit. The sampled fracture plane bears a chlorite-illite-rutile infill assemblage with fine-grained irregular-shaped monazite. Monazite from this sample produces a lower intercept age of 1376 ± 32 Ma (MSWD = 1.3) and is interpreted to represent the age of a major fluid flow event coeval with uplift along the Mount Isa fault. The monazite U-Pb geochronology presented in this study brackets the age of Cu mineralization and records the presence of multiple tectonic/hydrothermal events over the history of the deposit and enclosing host rocks.

Hot metamorphic complex in the Foreland Zone of the Variscan chain: insights from the Monte Filau orthogneiss (SW Sardinia), Italy

The Mt. Filau Orthogneiss belongs to the low-pressure (LP) - high-temperature (HT) Sulcis Metamorphic Complex located in the foreland zone of the Variscan belt (southern Sardinia). It consists of a core of orthogneiss and high metamorphic grade host rock tectonically juxtaposed below weakly- to non-metamorphic tectonic units, by the Tuerredda ductile shear zone. Our new petrographic and microstructural data on the fine–grained leucocratic facies of the Mt. Filau Orthogneiss are consistent with metamorphic peak under conditions of incipient partial melting in the sillimanite stability field at 680–700°C and 0.32–0.46 GPa. The retrograde P–T path occurred in the andalusite stability field, according to the growth of andalusite after sillimanite in leucocratic facies. The P-T conditions of metamorphic peak and incipient melting are both consistent with development, in the foreland zone, of a thermal anomaly accompanied by the upward rise of deep hot crust in the footwall of the Tueredda ductile shear zone. This transtensive shear zone acted as preferential channel for upward advection of deep hot melts during tectonically-driven isothermal decompression of middle crust. Therefore, we interpret the Sulcis Metamorphic Complex as the result of a tectonically controlled thermal perturbation, and underline the role of this late Variscan crustal scale tectonic structure in the development of hot metamorphic complex in an anomalous position within the foreland zone of the Variscan collisional belt.

A new UHP unit in the Western Alps: First occurrence of coesite from the Monviso Massif (Italy)

AbstractThe Western Alps are one of the most studied exhumed subduction-accretionary complexes worldwide. Ultrahigh-pressure (UHP) metamorphism has been documented there since the 1980s. We now report the first discovery of coesite in the meta-ophiolitic suite of the Monviso Massif, corresponding to the fourth UHP unit defined on the Western Alps. Previous petrographic studies and results from thermodynamic modeling already suggested that these Alpine units may have experienced UHP metamorphism, but no occurrences of index minerals, such as coesite, have been reported to date. The newly discovered coesite inclusions from the Monviso Massif occur as intact single crystals (10–60 µm) hosted by garnet. The observations suggest that they have escaped re-equilibration and maintained all the original features from the trapping time. The reduced size of the crystals and the lack of re-equilibration significantly differ from the typical textural features described in past findings (i.e., radial cracks, palisade texture of quartz surrounding coesite relicts). Detailed garnet inclusions analysis and thermodynamic modeling constrained the metamorphic peak conditions at P = 2.8–2.9 GPa and T = 500–520 °C within the coesite stability field.The Lago Superiore Unit represents the fourth UHP unit discovered on the Western Alps. The UHP metamorphism on the Western Alps was considered rare due to the escape process of unusual units from mantle depths. In our view, the implication of our discovery provides new insight into UHP processes that seem to be more common than expected. Further tectonic reconstructions should take into account the common features observed in the UHP units to better constrain the subduction- and exhumation-related mechanisms that drove the actual stacking of mountain belts.

Paleoproterozoic metagabbro xenolith in the southeastern Dom Feliciano Belt, southern Brazil: A new piece in the West Gondwana assembly

The eastern portion of the Dom Feliciano Belt in southernmost Brazil, known as Pelotas Terrane, is composed of Neoproterozoic rocks presenting metamorphic xenoliths in granitoid rocks, described and interpreted as “basement inliers”. Based on novel petrographic, mineral/whole rock/Sr–Nd geochemistry and U–Pb geochronological data, this paper presents the first record of a Paleoproterozoic metagabbro xenolith in the Pelotas Terrane. The studied xenolith exhibits a sharp and angular contact with the host rock and a predominant granoblastic texture, with sub-vertical mm-to cm-thick metamorphic banding, marked by the felsic and mafic bands, composed respectively of bytownite (An85) ± quartz, and diopside ± tremolite ± epidote. Zircon, apatite, and iron/titanium oxides occur as accessory minerals. The peak metamorphism for this rock is estimated at amphibolite facies conditions. The whole-rock chemistry of the metagabbro suggests a basic derivation (e.g., SiO2 52 wt%; Al2O3 12 wt%; Ni 33 ppm). The spidergrams demonstrate high LILE and HFSE concentrations. Chondrite-normalized REE patterns show enrichment in LREE. The Sr–Nd isotopic data show 87Sr/86Sr(i) = 0.701024, εNd(i) = 5.44, and Nd-TDM = 2.1 Ga. The metagabbro yielded a U–Pb Concordia age with analytical points arranged in a Discordia line. The upper intercept was interpreted as a magmatic crystallization age of 2.1 Ga (zircon cores), and the lower intercept indicated a resetting age of 616 Ma (zircon rims). The obtained Archean age (2.8 Ga) is interpreted as inherited. Gabbro's metamorphism likely reached the amphibolite facies, elemental and isotopic geochemistry. The remote zircon inheritance of xenoliths that have had polycyclic survival is used to link the occurrence of a gabbro xenolith, in the Pelotas Terrane (Dom Feliciano Belt; Southern Brazil), whose magmatic crystallization age is Paleoproterozoic, with the basement terranes before the Namaquan orogeny, in the Rehoboth region (Gariep Belt and Bushmanland/Namaqualand; Namibia). These units could be related to the evolution of an ancient Paleoproterozoic magmatic arc formed by the convergence of two Archean continents during the evolution of the Atlantica Supercontinent.

A New Alpine Metallogenic Model for the Pb-Ag Orogenic Deposits of Macôt-la Plagne and Peisey-Nancroix (Western Alps, France)

Understanding mass transfer associated with fluids circulation and deformation in the Alpine orogeny is often complex due to common multistage crystallization. For example, in two emblematic and historic Pb-Ag deposits of the French Alps, Macôt-la Plagne (MP) and Peisey-Nancroix (PN), a sedimentary or orogenic origin is still debated. To discriminate between the metallogenic models of the two deposits, an integrative methodology combining field, microstructural, mineralogical, thermobarometrical, and geochronological data was here applied for establishing detailed Pressure–Temperature–Time–Deformation (P-T-t-d) mineralization conditions. Both deposits are located in Permo-Triassic quartzite of the External Briançonnais domain along the Internal Briançonnais Front (Internal Western Alps). The ore mainly occurs as veins and disseminated textures containing galena, pyrite, and variable content of tetrahedrite–tennantite and chalcopyrite. Quartz porphyroclasts and sulfide microstructures indicate a dynamic recrystallization of the quartzite during the main fluid mineralization episode. Chlorites and K-white micas (phengite) chemical analysis and thermodynamic modeling from compositional maps indicate an onset of the mineralization at 280 °C, with a main precipitation stage at 315 ± 35 °C and 6.25 ± 0.75 kbar. In situ U-Pb dating on monazite, cogenetic with sulfides, gives ages around 35 Ma for both deposits. The integrative dataset converges for a cogenetic MP-PN Alpine Pb-Ag mineralization during deformation in relation to the thrusting of the “Nappe des Gypses” and the Internal Briançonnais at the metamorphic peak.

Implication of garnet zoning in high pressure- ultrahighpressure eclogite from Changning-Menglian suture zone, Bangbing area, Southeast Tibetan Plateau

Major and trace element zonation patterns were determined in ultrahigh-pressure eclogite garnets from the Changning-Menglian suture, Bangbing area. Several stages of eclogite metamorphism are recorded by garnet zonation. Cores of garnets are generally darker and inclusion-rich, while rims are brighter and inclusion-poor. Garnets were analyzed for the major element by EPMA and trace element by LA-ICP-MS. The results show that both major elements and rare earth elements in garnet have typical compositional bands. The major element zonation recorded the process of increasing temperature and pressure and the metamorphism of the eclogite from the core to rim, and the rare earth element zonation recorded different mineral chemical reactions during the formation of the garnet and its three stages of growth. Eclogites completely imply the process of rapid exhumation, cooling, and depressurization after short-term peak metamorphism. The contents of some trace elements (such as Y and Sr) in garnet are controlled by major elements and mineral combinations. The formation of new minerals in the process of retrograde metamorphism affects the distribution of trace elements.

Constraint on the temperature of A-type magma from contact metamorphic aureole, Biesituobie batholith, West Junggar in NW China, Central Asian Orogenic Belt

The Biesituobie A-type batholith in West Junggar in NW China of the Central Asian Orogenic Belt contains metapelite xenoliths derived from the contact metamorphic aureole. These xenoliths could be divided into two types: biotite-muscovite-andalusite hornfels and cordierite-alkaline feldspar-sillimanite-corundum gneiss, indicating prograde metamorphism. The phase equilibrium modeling on the cordierite-alkaline feldspar-sillimanite-corundum gneiss sample yielded a peak condition of P = 3.0−4.0 kbar, T = 760−800 °C. Similarly, the Na-in-cordierite geothermometer yielded a peak condition of T = 771−780 °C. The temperature of the peak metamorphism could be considered as a lower limit of the temperature of the Biesituobie A-type batholith. On the contrary, the Ti-in-zircon geothermometer applied to the Biesituobie A-type batholith zircons yields a mean temperature of 672 ± 22 °C. It suggested that the application of Ti-in-zircon geothermometer on A-type magma may involve a >100 °C underestimation. Zircon U-Pb dating indicates that the age of the contact metamorphism is between 263 and 286 Ma, consistent with the age of the pluton at 274 Ma. The result of 760−800 °C calculated from the contact metamorphic aureole of the Biesituobie batholith put a lower limit on considering the temperature condition of the A-type granite series from a metamorphic constraint.

Modern style nappe stacking in the Paleoproterozoic lower crust: An example from the snowbird tectonic zone, Canadian Shield

The Intra Tantato shear zone (ITSZ), which occurs within the East Athabasca mylonite triangle in the southern portion of the ∼1.9 Ga Snowbird tectonic zone in northern Saskatchewan, Canada, is a low to high angle, curvi-planar, kilometer-thick mylonite zone hosted within granulite-facies gneiss that records a complex history with both thrust- and normal-sense motion. New field mapping, microstructural analysis, and quartz crystallographic preferred orientations (CPO) detail a composite history for the ITSZ that includes top-to-the-ENE thrusting of the hanging wall over the footwall that is overprinted by normal-sense top-to-the-WSW reactivation. Quartz CPOs further indicate constrictional deformation related to both events and quartz c-axis opening angles define deformation temperatures of ∼750 ± 50 °C for the thrusting event and ∼550 ± 50 °C for the later, normal-sense reactivation.Monazite and garnet geochronology on rocks within and around the ITSZ indicate it may have nucleated along a pre-existing 2.6–2.5 Ga sub-horizontal fabric. This appears to have occurred immediately after the ∼1.920 Ga metamorphic peak of the hanging wall and during the ∼1.905 Ga metamorphic peak of the footwall. Monazite ages from small matrix grains in interstitial positions in specimens that record reactivation kinematics indicate overprinting occurred at ∼1.860–1.830 Ga, contemporaneous with the earliest recorded activity along the nearby Grease River Shear Zone.Overall, this tectonic evolution can be attributed to a crustal thickening process that occurred in the Athabasca mylonite triangle during the Snowbird orogeny at ∼1.9 Ga and during the post-collisional exhumation processes. Moreover, these results show that the lower continental crust can actively accommodate shortening during large-scale collisional events with tectonic units piling up via nappe-stacking processes similarly to what is typically observed in the middle crust.

The depositional environment of the Koeris Formation in the Aggeneys-Gamsberg ore district, South Africa

Abstract The Koeris Formation in the Mesoproterozoic Aggeneys-Gamsberg ore district (South Africa) is located in the hanging wall of an unconformity, only metres above the giant Gamsberg and Big Syncline Pb-Zn ore deposits and potentially contains critical information on the metallogenesis and its geodynamic setting. We, therefore, conducted geochemical analyses of amphibolite and metasedimentary rocks of this formation in the core of the Gamsberg fold, the results of which shed new light on the plate tectonic processes after deposition of the sedimentary exhalative ore. Major elements previously used for the characterisation of the amphibolite and the discrimination of the plate tectonic setting of its protolith had been mobilised during retrograde metamorphic overprint and are thus of little tectonic significance. More informative are certain trace elements: The amphibolite shows a strong depletion in Nb, Ta and P, typical of subduction-related magmatism, whereas exceptionally strong enrichment in mobile elements like Rb and Ba indicate the participation of continental material (crust/sediment) in the enrichment of the mantle wedge. This is in perfect agreement with existing U-Pb age data, which attest formation of the mafic metavolcanic rocks of the Koeris Formation at the very end of the Okiepian orogenesis (1 210 to 1 180 Ma), when subduction was terminated. Generally, very high Zn and Pb contents speak for metasomatic interaction of the Koeris Formation with the Zn-and Pb-rich Gamsberg deposit prior to peak metamorphism during the Klondikean orogeny (1 040 to 1 020 Ma). The fact that especially the immobile trace elements (e.g. Nb, Ta) in all metasedimentary rocks mirror the chemical characteristics of the metavolcanic rocks indicates local amphibolite detritus as a main source of the sediments. We conclude that the metamorphic volcano-sedimentary sequence of the Koeris Formation is not, as previously assumed, the product of a large back-arc basin but was deposited in response to tectonic uplift in a small-scale intra- or inter-montane basin at the very end of the Okiepian orogeny. The thick amphibolite layers concentrated within this topographic depression may have played a crucial role in the conservation of the underlying ore. Therefore, the occurrence of the Koeris Formation may indicate favorable locations for further exploration.

Ultrahigh-density carbonic fluids in ultrahigh-temperature metagabbros from the Palghat-Cauvery Suture Zone, Southern India: Phase equilibria modelling and fluid inclusion study

Fluid inclusions trapped in high-grade metamorphic minerals offer direct evidence for the nature and role of fluids involved in lower crustal processes. Here, we report extremely high-density (~1.155 g/cm 3 ) carbonic fluid inclusions preserved within garnet grains in metagabbros from the central segment of the Palghat-Cauvery Suture Zone (PCSZ) in southern India. The metagabbros are composed of coarse-grained garnet, clinopyroxene, orthopyroxene, plagioclase, quartz, and retrograde calcic amphibole. Garnet occurs either as porphyroblastic grains formed during prograde to peak stage, or as fine-grained aggregates around plagioclase possibly formed by post-peak decompressional (near-isobaric) cooling. The peak and retrograde P–T conditions have been estimated as ~940 °C/~11.5 kbar and ~ 840–850 °C/~9.3–10.2 kbar, respectively, based on phase equilibria modelling using pseudosection computation. Fluid inclusions occur as primary and secondary phases within the porphyroblastic garnet and matrix plagioclase. The melting temperatures of these fluid inclusions are in the range of −57.4 to −56.6 °C, indicating nearly pure CO 2 composition. The lowest homogenization temperature from the primary inclusions in garnet (−50.1 °C) translates to extremely high density of 1.155 g/cm 3 . The calculated isochores for the inclusions (~11.3 kbar at 950 °C) are broadly consistent with the peak P–T conditions, suggesting that the fluids are trapped near the peak metamorphic stage under a dry condition. The preservation of ultrahigh-density carbonic inclusions is probably due to isochore-parallel decompressional cooling after the peak metamorphism. The secondary inclusions show slightly lower densities of 1.014–1.053 g/cm 3 (in garnet) and 0.955–0.976 g/cm 3 (in plagioclase), the isochores of which suggest a lower-pressure range of 4.8–5.8 kbar at 800 °C. The high-density pure CO 2 fluids reported in this study as well as in previous studies from the PCSZ, and the isochoric evolution provide robust evidence for the presence of supercritical carbonic fluids in the lower continental crust. • Extremely high-density (~1.155 g/cm 3 ) carbonic fluid inclusions in metagabbros. • Peak and retrograde conditions of ~940 °C/~11.5 kbar and ~ 840–850 °C/~9.3–10.2 kbar. • Garnet aggregates around plagioclase formed by post-peak near-isobaric cooling. • Isochore-parallel decompressional cooling after the peak metamorphism. • Ultrahigh-density fluids are preserved due to isochoric exhumation path.

Petrology and P-T-t Path of Huangyuan Group and Maxianshan Group in the Central Qilian Block, NW China: Implications for Tectonic Evolution of the Proto-Tethys Ocean

AbstractThe Qilian orogen marks the northern boundary of the Proto-Tethys Ocean and consists of the North Qilian belt, the Central Qilian block and the South Qilian belt. The metamorphic P–T–t path of the Central Qilian block and the tectonothermal processes that affected it are crucial to understanding the closure of the ocean. This paper presents a systematic investigation of the metamorphic P–T–t path of amphibolites from the Precambrian Huangyuan Group and Maxianshan Group in the Central Qilian block. An amphibole 40Ar/39Ar age of 456.7 ± 5.2 Ma from a garnet amphibolite and a 427.8 ± 3.8 Ma muscovite 40Ar/39Ar age from an orthogneiss in the Huangyuan Group, and amphibole and biotite 40Ar/39Ar ages of 408.7 ± 2.2 Ma and 349.9 ± 2.0 Ma from an amphibolite and a 361.4 ± 2.0 Ma biotite 40Ar/39Ar age from an orthogneiss in the Maxianshan Group record cooling stages after peak metamorphism. Phase equilibrium modeling of all amphibolite samples indicates clockwise P–T paths. A garnet amphibolite from the sillimanite zone in the Huangyuan Group recorded peak metamorphic conditions of 5.6–6.6 kbar and 730–770°C, reflecting granulite-facies metamorphism. Peak P–T conditions for a garnet amphibolite from the garnet zone are constrained to 580–600°C and 6–7 kbar. The amphibolite sample from the Maxianshan Group indicates peak P–T conditions of ~5.2 kbar and ~630°C. The Central Qilian block was affected by both southward subduction of the North Qilian Ocean and northward subduction of the South Qilian Ocean, which resulted in paired metamorphic belts along the northern margin of the Central Qilian block and accretion of the Hualong block to its southern margin. The subducted North Qilian oceanic slab detached and the remainder was exhumed at 453–445 Ma, accompanied by the extension and retrograde metamorphism of the overlying crust of Huangyuan Group at 457–428 Ma. Subsequent cooling of the Maxianshan Group indicates that the thermal regime relaxed during post-collisional extension at c. 409–350 Ma between the Hualong block and the Central Qilian block.

Eocene to Miocene metamorphic evolution and tectonic implication of the Ilam Nappe in Nepal Himalaya: Constraints from P–T conditions and monazite petrochronology

The Ilam Nappe, consisting of the High Himalayan Crystalline Sequence (HHCS) in far-eastern Nepal, was largely emplaced southward along the Main Central Thrust (MCT). Here we present the P–T–t path of the Ilam Nappe using conventional geothermobarometers, pseudosection modelling, and monazite petrochronology. Amphibole-bearing migmatite just above the MCT experienced prolonged fluid-present melting from prograde metamorphism at 39–31 Ma to peak upper amphibolite-facies metamorphism (694 °C and 10.3 kbar) during 24–19 Ma, followed by cooling. Kyanite-sillimanite migmatites record prograde metamorphism at 35–28 Ma and peak lower granulite-facies metamorphism (738–765 °C and 8.3–9.7 kbar) under kyanite stability via the muscovite dehydration reaction at 25–20 Ma, followed by isothermal decompression under sillimanite stability. The clockwise P–T path and Early-Middle Miocene peak metamorphism of the Ilam Nappe can be correlated to those in the lower-middle HHCS hinterland. The Eocene-Oligocene prograde metamorphism with a low geothermal gradient of 15–25 °C/km in the Ilam Nappe and the entire HHCS hinterland could represent the earlier crustal thickening accompanied by large-scale overthrusting before the normal faulting of the South Thrust Detachment System (STDS), followed by diachronous extrusions of the upper HHCS along the High Himal Thrust, and of the lower-middle HHCS along the MCT with nappe emplacement. The lower granulite-facies Ilam Nappe was exhumed during the Early-Middle Miocene from the deeper or lower structural level of the HHCS, compared to the earlier Oligocene exhumation of the upper amphibolite-facies Karnali and Kathmandu klippes from the shallower or uppermost structural level of the HHCS.

Incipient collision of the Rae and Slave cratons at ca. 1.95 Ga

AbstractIncipient collision between the Rae and Slave cratons represents a critical initial stage in the final assembly of the composite Laurentian craton. However, the timing and kinematics of this event remain poorly constrained. Here, we integrate phase equilibria modeling and Lu-Hf garnet and U-Pb monazite and zircon petrochronology to precisely date the timing of peak metamorphism along the Great Slave Lake shear zone, the central segment of the western Rae cratonic margin that was the locus of terminal Rae-Slave collision. Zircon and monazite U-Pb ages broadly constrain prograde-to-peak metamorphism to 1933–1913 Ma, whereas the timing of upperamphibolite facies peak metamorphism (0.8–1.0 GPa and 750–800 °C) is best constrained by two garnet Lu-Hf ages of 1931 ± 12 and 1917 ± 6 Ma. Our results highlight the inherent strengths and limitations of garnet, monazite, and zircon petrochronology in the context of dating Paleoproterozoic-aged metamorphism. Based on the timing of crustal thickening constrained by Lu-Hf garnet geochronology, we posit that the Slave craton first collided with the western Rae margin at ca. 1950 Ma. Taken together with prior constraints on the timing of metamorphism elsewhere along the western Rae margin, we propose incipient collision was followed by the northwards zippering of the margins and the eventual ~500 km of dextral translation of the Slave craton along the Great Slave Lake shear zone.

Multi‐episodic formation of baddeleyite and zircon in polymetamorphic anorthosite and rutile‐bearing ilmenitite from the Chiapas Massif Complex, Mexico

AbstractMassif‐type anorthosite and comagmatic associations of rutile‐bearing ilmenitite (RBI) and oxide‐apatite‐rich amphibolite (OARA) from the Chiapas Massif Complex (CMC) in southeastern Mexico display a protracted billion‐year accessory mineral record encompassing magmatic crystallization at c. 1.0 Ga to recent ductile shear deformation at c. 3.0 Ma. Multiple discrete zircon populations between these age end‐members resulted from neoformation/recrystallization during local to regional metamorphism that affected the southeastern portion of the CMC. The ubiquitous presence of relict baddeleyite (ZrO2), along with various zircon generations spatially associated with pristine to partly retrogressed Zr‐bearing igneous and metamorphic minerals (e.g., ilmenite, rutile, högbomite and garnet), suggests significant Zr diffusive re‐equilibration (exsolution) during slow cooling and mineral breakdown followed by crystallization of baddeleyite. The subsequent transformation of baddeleyite into zircon was likely driven by reaction with Si‐bearing fluids in several geochronologically identified metamorphic stages. Strikingly contrasting compositional signatures in coeval zircon from anorthosite (silicate‐dominated) and comagmatic RBI (Ti‐Fe‐oxide‐dominated) indicate a major role of fluids locally equilibrating with the rock matrix, as indicated by distinct zircon trace element and oxygen isotopic compositions. A high‐grade metamorphic event at c. 950 Ma is likely responsible for the formation of coarse‐grained rutile (~0.1–10 mm in diameter), srilankite, zircon and garnet with rutile inclusions as well as metamorphic högbomite surrounding Fe‐Mg spinel. Zr‐in‐rutile minimum temperatures suggest >730°C for this event, which may correlate to rutile‐forming granulite facies metamorphism in other Grenvillian‐aged basement rocks in Mexico and northern South America. A younger generation of baddeleyite exsolution occurred during post‐peak cooling of coarse‐grained rutile, reflected in rimward Zr depletion and formation of discontinuous baddeleyite coronas. Baddeleyite around rutile was then transformed into zircon possibly during subsequent metamorphism at c. 920 or 620 Ma, resulting from syn‐kinematic and contact metamorphism, respectively. Regional metamorphism at c. 450 and 250 Ma extensively overprinted the existing zircon population, especially during the Triassic event, as suggested by a significant presence of zircon with this age. Nearly pristine baddeleyite occurring interstitial to ilmenite yielded an isochron age of c. 232 Ma according to in situ U–Pb secondary ion mass spectrometry (SIMS), suggesting either formation during metamorphic peak conditions or post‐peak cooling. Zircon with ages of c. 80–100 Ma in anorthosite is identified for the first time within the CMC and coincides with cooling ages of c. 100 Ma for coarse‐grained rutile. This age is similar to those of rocks occurring ~200 km further to the east in Guatemala, which are also bounded to the Polochic fault system but overprinted by eclogite facies metamorphism. A high‐pressure event in the southern CMC after 200 Ma, however, is presently unsupported. Although the abundance of rutile and ilmenite is unusually high in the CMC anorthosite assemblage compared with common igneous rocks, the reactions documented here nonetheless stress the importance of these phases for generating Zr‐bearing accessory minerals over a wide range of metamorphic conditions.

Structural and kinematic analysis of the Cuonadong dome, southern Tibet, China: Implications for middle-crust deformation

The Cuonadong Dome is located at the eastern part of the North Himalayan gneiss domes. Structural and kinematic analysis in the middle unit of the Cuonadong Dome provide significant new insights into the structural deformation of the middle crust in the Northern Himalaya. Field observations, microscopic kinematic and EBSD analyses, and deformation temperature estimates from the mid-crustal rocks of the Cuonadong Dome show high-strain D2 (top-to-N) kinematic fabrics with a downward-to-north progression from dominantly top-to-north shear in the garnet zone, to solely top-to-north shear in the kyanite/sillimanite zone. A combination of mineral assemblages, microstructures, and quartz crystal preferred orientation patterns indicates deformation temperatures associated with D2 kinematic fabrics increase with structural depth from ∼450 °C in garnet zone to > 630 °C in the kyanite/sillimanite zone. These values broadly overlap temperature range deduced from metamorphic petrography (450–630 °C). Microtextural relations indicate that peak metamorphism occurred post-D1 and pre-D2 deformation. Combined with the ∼1000-m-thick ductile deformation zone, the interpreted deformation temperatures yielded an average thermal field gradient of ∼ 180 °C/km over 1.05 km of structural thickness. Based on the geochronologic data, midcrustal D2 extensional deformation in the Cuonadong Dome initiated as early as early Oligocene (32 Ma) and ended during the middle Miocene (∼14 Ma). Comparative studies of D2 ductile deformation in midcrustal rocks from other North Himalayan gneiss domes and the South Tibetan Detachment System indicate that D2 shear zone in the Cuonadong Dome represent the northern part of the South Tibetan Detachment System, and highlight temporal and spatial variation in midcrustal D2 deformation patterns over the southern Tibet during the Himalayan orogeny.

Multi‐stage metamorphism of ultrahigh‐temperature Mg‐Al granulites during Gondwana assembly: Evidence from southern India

The Southern Granulite Terrane (SGT) in India hosts granulite facies rocks metamorphosed at ultra‐high temperature (UHT) conditions in the various crustal blocks, as well as within the Palghat‐Cauvery Suture Zone (PCSZ), that is considered as a trace of the Late Neoproterozoic—Cambrian Gondwana suture. Here we investigate UHT granulites from the northern margin of the Madurai Block adjacent to the PCSZ where Mg‐Al‐rich granulites are exposed. We identify sodic gedrite + kyanite in these rocks as the high‐pressure prograde stage assemblage, followed by sillimanite‐garnet‐orthopyroxene that formed during pressure decrease and temperature increase. The rare remnant gedrite is also stable at the near‐peak UHT metamorphism until it was replaced by sapphirine. The rocks subsequently underwent decompression that formed sapphirine + cordierite and sapphirine + plagioclase symplectite around sillimanite. Dehydration during decompression generated orthopyroxene‐sillimanite‐quartz assemblage with the appearance of sapphirine, defining the diagnostic mineral assemblage indicative of peak UHT metamorphism (T > 900°C) at relatively high‐pressure (P > 9 kbar). The UHT peak metamorphism in this region is consistent with the results of P–T calculations using conventional geothermometers and phase equilibrium modelling (T up to 1,050°C, P over12 kbar). Zircon and monazite geochronology on the UHT metapelites indicate distinct stages. Detrital zircon grains in the metasediments indicate protolith from ca. 2.5 Ga igneous source and the metamorphic overgrowths yield 206Pb/238U mean ages concentrated at ca. 550–520 Ma. Monazite ages define another younger group 206Pb/238U mean ages at ca. 450 Ma. The prograde high‐pressure granulite‐facies metamorphism and following UHT event correlate with the subduction‐collision tectonics at 550–500 Ma associated with the final stage of amalgamation of the Gondwana supercontinent, while the 420–460 Ma monazite age records a later hydration at the post‐orogenic stage, possibly associated with deep shearing and fluid influx.