Biotite Dehydration Melting Research Articles

Evidence of High‐Pressure Metamorphism Along the Mahanadi Shear Zone in the Eastern Ghats Province, Eastern India: Implications on Tectonics and Continental Assembly Involving India and East Antarctica

ABSTRACTA suite of mafic granulite enclaves within mylonitised felsic gneiss occurring along the E‐W trending Mahanadi Shear Zone of the Eastern Ghats Province preserves evidence of high‐pressure metamorphism. Garnet‐clinopyroxene‐bearing mafic granulite contains a mineral assemblage of garnet + clinopyroxene + plagioclase + quartz + rutile which was formed after dehydration melting of a hornblende‐bearing protolith during M1 metamorphism that peaked at 1.1–1.4 GPa, 760°C–840°C. The retrograde stage (M1R) is marked by the formation of hornblende and symplectic intergrowth of clinopyroxene + plagioclase + orthopyroxene after garnet at 0.8–0.9 GPa, 760°C–810°C, suggesting an isothermal decompression type P–T path. The whole rock trace element and REE characteristics suggest a MORB‐OIB protolith for the mafic granulites. The host felsic gneiss has a granitic protolith which was emplaced in an arc setting. The rocks exposed south of the Mahanadi Shear Zone in the Phulbani domain are represented by granulites with contrasting metamorphic characteristics. The garnet‐orthopyroxene‐bearing mafic granulite within coarse‐grained charnockite and the aluminous granulite within felsic gneiss show evidence of biotite dehydration melting. The peak M1 assemblage in the aluminous granulite is represented by the assemblage spinel + garnet + quartz + plagioclase + K‐feldspar which was stable at 0.70–0.74 GPa, 904°C–935°C. M1R in this rock is characterised by coronas of garnet and sillimanite over spinel and the formation of matrix biotite at 707°C–806°C by near‐isobaric cooling. Similar isobaric cooling has been documented from the formation of garnet, clinopyroxene and quartz coronas on orthopyroxene in mafic granulite and garnet and quartz coronas on clinopyroxene, wollastonite and calcite in calc‐silicate granulite. The juxtaposition of lower crustal rocks showing clockwise and counterclockwise P–T paths across the Mahanadi Shear Zone implies a paired metamorphic character in a subduction–collision setting. Zircon U‐Pb and monazite U‐Th‐total Pb data show a complex history of the rock suite. The enclave suite of rocks within the Mahanadi Shear Zone underwent peak M1 metamorphism at ca. 980–960 Ma which was followed by decompression to a shallower level by ca. 960 Ma when the host granitic magma crystallised. Rocks occurring in the Phulbani domain (southernly placed crustal domain), on the other hand, underwent ultrahigh temperature metamorphism at shallower crustal levels broadly at the same time. We argue that the southern Phulbani domain of the Eastern Ghats Province, India, collided with the Angul‐Prydz domain of the Rayner Province, East Antarctica which eventually caused underthrusting of the former below the latter across the Mahanadi Shear Zone. In the context of the Eastern Ghats‐Rayner reconstruction, this indicates the closure of the intervening Mawson Sea. A second metamorphic event (M2) reworked the exhumed deep crustal rocks at ca. 900 Ma during the final docking of the Eastern Ghats‐Rayner belt against cratonic India. Our results clearly show that the Angul domain is an exotic block, and the Mahanadi Shear Zone is a terrane boundary shear zone suturing discrete domains of the Rayner‐Eastern Ghats orogen.

Bayesian frameworks for integrating petrologic and geochronologic data

Constraining the absolute time and duration of geologic processes is one of the great challenges and goals in Earth sciences. Increasingly, the integration of geochronologic constraints with petrologic information is being qualitatively applied to understanding the timescales of metamorphic, igneous, tectonic, and fluid-related processes. Many rocks and geochronometers preserve relative age constraints such as compositional zoning or cross cutting relationships. This prior information can be formalized in a Bayesian statistical framework to generate a probabilistic posterior chronology. As we show here, these “age-sequence” models can enhance precision on geochronologic dates and rates and insight into tectonic models. Bayesian modeling of complex, concentrically, zoned monazite from the northern Appalachian orogen was used to develop a detailed temperature-time history through the Acadian (∼405–395 Ma) and Neoacadian (∼380–350 Ma) orogenies with significantly reduced uncertainties (40–70 %). Modeling of zoned monazite from a southern Trans-Hudson orogen granulite yielded durations of 0.5+9/-0.4 Ma and 20+5/-8 Ma for biotite-dehydration melting and suprasolidus conditions, respectively. The relatively short intervals of heating and peak conditions are consistent with a back-arc tectonic setting. A complementary approach, Bayesian change point detection, provides a framework to constrain the timing of compositional changes that can be linked with metamorphic reactions. Applying this approach in the northern Appalachian orogen demonstrates contrasting durations of low-Y monazite crystallization (∼10 vs ∼30 myr) in regions with different pressure-temperature histories. Compositionally distinct monazite domains can be linked with garnet stability, which provides a key constraint on tectonic models. Bayesian statistical analysis represent a powerful tool that can be widely applied to refine the absolute time and duration of geologic processes. A more objective and reproducible set of interpretations are produced by this more formal, although not necessarily complex, statistical analysis.

Evolution of the rare-metal mineralization system associated with collision-related pegmatites in the western Altyn Tagh Orogen, Tugeman, NW China

On a regional scale, rare-metal pegmatite groups (pegmatites of common origin) display a zonal distribution with different chemical compositions. Whether the regional distribution of such pegmatites dike in a convergent zone is related to dehydration melting of different micas remains unclear. In recent years, the Altyn Tagh Orogen (ATO) in the northwest China has gained attention due to economically significant rare-metal mineralization. Our study focuses on the geochronology of columbite-group minerals (CGMs) and geochemistry of tourmaline from the Be-mineralized pegmatites in the Tugeman and Ayage Be deposits. CGM dating shows that these Be-mineralized pegmatites formed between 485 and 484 Ma. Subsequent hydrothermal activity, occurring around 453–450 Ma, is related to the development of a ductile shear zone. Evidence from tourmaline mineralogy and major elements reveals that the evolution of the Be-mineralized pegmatite is influenced by crystallization differentiation and subsequent fluid exsolution. Tourmalines from these pegmatites exhibit B isotope compositions ranging from –12.2 ‰ to –14.8 ‰, indicating a crustal metasediment source. Based on these findings, we propose a model for the rare-metal mineralization system in the Tugeman area: the sequential formation of Be to Li-mineralized pegmatites likely results from dehydration melting of muscovite and biotite during prolonged subduction and collision between the South Altyn Subduction–Collision Belt (SAB) and Central Altyn Block (CAB).

Li-mineralized pegmatite of anatectic origin: Constraints from Li[sbnd]Hf isotopes and geochemical modeling

Pegmatites are important reservoirs of lithium resources. There is an ongoing debate as to whether Li-mineralized pegmatites were generated from evolved granitic systems via extreme differentiation or by direct melting of metasedimentary rocks. The Chinese Altay is one of the largest pegmatite provinces in the world, and includes many rare-metal pegmatites (e.g., Koktokay, Kelumute, Kaluan and Azubai). However, they display significant differences in formation ages and isotopic compositions compared with the neighbouring or potential parental granites. In this contribution, we present a case study of the Kaluan Li ore deposit in the Chinese Altay, elucidating the origins of pegmatites and related Li mineralization. The Kaluan Li-mineralized pegmatites are characterized by high SiO2, K2O, Na2O, Li, Cs, Ta, and Rb contents; low Fe2O3T, MgO, Ba, and Sr contents; and peraluminous signatures. They have homogeneous HfLi isotopic compositions (εHf (t) = −0.6 to +2.5; δ7Li = +1.6 to +5.8‰) that are within the range of HfLi isotopic compositions of the Kulumuti Group schists (εHf (t) = −2.8 to +7.5; δ7Li = −8.6 to +10.8‰). In addition, the chemical compositions of the melt derived from the partial melting of the Kulumuti Group by phase equilibrium modeling are compatible with our analyses of the Kaluan Li-mineralized pegmatites. These similarities suggest that the Kaluan Li-mineralized pegmatites were derived from the partial melting of the Kulumuti Group schists at depths during the Triassic, in a post-orogenic and extensional setting. The generation of these pegmatites was mostly controlled by biotite dehydration melting at relatively high temperatures, rather than by the low-temperature melting of muscovite. Trace elements modeling results show that approximately 33–36% partial melting of the average Kulumuti Group schist may produce melts with maximum Li concentrations ranging from 664 to 751 ppm. Subsequently, a high degree of fractionation exceeding 85%, was required for the Li concentration of the residual melt to reach concentrations suitable for crystallization into albite-spodumene pegmatites. Based on our research, an anatectic origin model may have general applicability in explaining the genesis and mineralization of rare-metal pegmatites in the Chinese Altay.

Genesis of the Ke’eryin Two-Mica Monzogranite in the Ke’eryin Pegmatite-Type Lithium Ore Field, Songpan–Garze Orogenic Belt: Evidence from Lithium Isotopes

Previous studies on the Ke’eryin pegmatite-type lithium ore field in the Songpan–Ganzi Orogenic Belt have explored the characteristics of the parent rock but have not precisely determined its magma source area. This uncertainty limits our understanding of the regularity of lithium ore formation in this region. In this study, to address the issue of the precise source area of the parent rock of lithium mineralization, a detailed analysis of the Li isotope composition of the ore-forming parent rock (Ke’eryin two-mica monzogranite) and its potential source rocks (Triassic Xikang Group metamorphic rocks) was conducted. The δ7Li values of the Ke’eryin two-mica monzogranite, Xikang Group metasandstone, and Xikang Group mica schist are −3.3–−0.7‰ (average: −1.43‰), +0.1–+6.9‰ (average: +3.83‰), and −9.1–0‰ (average: −5.00‰), respectively. The Li isotopic composition of the Ke’eryin two-mica monzogranite is notably different from the metasandstone and aligns more closely with the mica schist, suggesting that the mica schist is its primary source rock. The heavy Li isotopic composition of the two-mica monzogranite compared to the mica schist may have resulted from the separation of the peritectic garnet into the residual phase during the biotite dehydration melting process. Moreover, the low-temperature weathering of the source rocks may have been the main factor leading to the lighter lithium isotope composition of the Xikang Group mica schist compared to the metasandstone. Further analysis suggests that continental crust weathering and crustal folding and thickening play crucial roles in the enrichment of lithium during multi-cycle orogenies.

Recognition of Neogene tin mineralization in the Southeast Asian tin belt

Abstract The Southeast (SE) Asian tin belt is a major tin producer globally, with a prolonged mineralization history from the Permian to Paleogene (ca. 285–45 Ma). Tin deposits in this region are typically associated with tectonic settings that involve subduction and collision of the Paleo-, Meso-, and Neo-Tethys slabs. Ca. 40 Ma, a notable transition occurred in the tectonic regime of SE Tibet, with the Neo-Tethys subduction giving way to lateral extrusion of the Indochina block along major strike-slip faults. Previously, it was believed that this shift had brought tin mineralization to a halt. In this study, we present in situ laser ablation–inductively coupled plasma–mass spectrometry U-Pb cassiterite ages of 21–19 Ma from the Yunling tin deposit located in western Yunnan, China. Yunling produces gem-quality cassiterite that is transparent but contains low U contents, which renders usual U-Pb dating techniques unusable. To address this, a customized dating protocol involving cathodoluminescence (CL) imaging and testing of U distribution on crystallographically well-defined cassiterite crystals was applied. The study revealed two types of primary microstructures in cassiterite: volumetrically dominant concentric oscillatory growth zones and subordinate CL-dark sector zones. The U content shows a preferential enrichment in the CL-dark sector zones, typically tens of parts per million (ppm), which is two orders of magnitude greater than the U content in the concentric zone. This is significant, as the dating results (21–19 Ma) obtained through the targeted selection of CL-dark sector zones represent the youngest tin mineralization event in the SE Asian tin belt. Additionally, these results challenge the established belief that the ore-hosting Indosinian granite, dated to ca. 215 Ma, was related to Sn mineralization in the Yunling area. We suggest that emplacement of the early Neogene Sn mineralization at Yunling may be genetically connected to extensive delamination of the lithosphere in southeastern Tibet. The lithospheric delamination led to the upwelling and decompression partial melting of the asthenosphere, which provides a plausible explanation for the high temperature required for the release of Sn from biotite-dehydration melting of sedimentary protolith. The study also highlights the potential of Oligocene–Miocene-aged granites situated in the Sanjiang Tethys and adjacent areas of western Yunnan as prospective exploration targets for tin mineralization.

The Grenville Province: revisiting the orogenic framework and integrating recent findings

The Grenville Province holds record of Mesoproterozoic orogenic processes at the Laurentian margin during the assembly of Rodinia. This contribution reviews key characteristics of the Province, integrates recently published data, and questions some earlier interpretations. Despite crustal reworking during the Grenvillian orogeny (1.09–0.98 Ga), contrasts between crustal-age domains across the Allochthon Boundary largely reflect differences between internal and external Laurentia, and allochthonous units were far-travelled in the west but not in the east. In addition, there is growing evidence towards a continuum between the Ottawan and Rigolet orogenic phases. Based on ages of metamorphism, the High-Pressure segments of the hinterland reflect localized thickening at the front of the Ottawan orogen and evidence for an orogenic plateau is limited to the western Grenville. Peak-metamorphic conditions in the Mid-P orogenic core, mostly within the confines of biotite dehydration melting (max ~900oC) for aluminous rocks, culminated in early Ottawan and were followed by protracted extension. In addition, high-temperature granitoid magmatism was active throughout the duration of the orogeny and localized in formerly pericratonic terranes accreted at ~1.4 Ga and 1.2–1.1 Ga, indicating an influence of inherited lithospheric structures on Grenvillian mantle dynamics. The overall orogenic pattern reveals considerable lateral variations in the hinterland potentially linked to the earlier configuration of external Lautentia and is consistent with weak lithosphere supporting a broad, hot, but not necessarily high-standing orogen for most of the Ottawan, in contrast to the Rigolet phase that marks the propagation of the orogen against Archean lithosphere of internal Laurentia.

Partial melting mechanisms of peraluminous felsic magmatism in a collisional orogen: An example from the Khondalite belt, North China craton

AbstractSedimentary‐derived (S‐type) granites are an important product of orogenic metamorphism, and a range of subtypes can be recognized by differences in field occurrence, mineralogy and geochemistry. These subtypes can reflect variations of initial protolith composition, partial melting reactions, pressure and temperature of anatexis, or magmatic processes that occur during ascent through the crust (e.g. mineral fractional crystallization or crustal assimilation). Together, these diverse factors complicate geological interpretation of the history of peraluminous felsic melt fractions in orogenic settings. To assess the influence of these factors, we performed integrated field investigation, petrology, geochemistry, geochronology and phase equilibrium modelling on a series of leucosomes within migmatite associated with different S‐type granites within the Khondalite belt, North China craton (NCC), which is an archetypal collisional orogen. Three types of leucosome are recognized in the east Khondalite belt: leucogranitic leucosome, K‐feldspar (Kfs)‐rich granitic leucosome and garnet (Grt)‐rich granitic leucosome. Phase equilibrium modelling of partial melting and fractional crystallization processes indicate that the leucogranitic leucosomes were mostly produced through fluid‐present melting, Kfs‐rich granitic leucosomes are produced through muscovite dehydration melting with 3 vol.% garnet fractional crystallization, and Grt‐rich granitic leucosomes are produced through biotite dehydration melting with 20–40 vol.% K‐feldspar fractional crystallization and up to 20 vol.% peritectic garnet entrainment. Mineral fractional crystallization and peritectic mineral entrainment occur in the source during melting, and play equally important roles in partial melting mechanisms in terms of affecting the geochemical compositions of granitic melts. Thus, we suggest that peraluminous felsic magmas preserved in collisional orogens are dominantly produced by fluid‐absent melting in the middle to deep continental crust, although extraction of low‐volume melt fractions from an anatectic source region at shallower depths during fluid‐present melting can also generate small amounts of S‐type granites that subsequently crystallize at high structural levels in the crust.

The large granitic pegmatite Li deposits formed at final collision of a three micro-continent amalgamation in the Central Altun Tagh, Northwest China

Micro-continent collision and amalgamation play pivotal roles in continental convergence and contribute significantly to continental crustal growth. However, the understanding of multiple micro-continent amalgamation processes, particularly their influence on the formation of large Li-Be deposits, remains limited. The Altun orogenic system, resulting from the collision and amalgamation of three micro-continents in the Proto-Tethys Ocean, has recently revealed numerous granitic pegmatite-type Li-Be deposits, presenting an ideal opportunity to investigate the mineralization. Our research focused on two large granitic pegmatite Li deposits, South Washixia and Tamuqie in the Central Altun Tagh, Northwest China. Our findings suggest: 1) The structural and photomicrograph characteristics observed in both the Tamuqie and South Washixia granitic pegmatite Li deposits, indicate that the pegmatites emplaced in shear zone within a contractional deformation zone. 2) The age of the large granitic pegmatite Li deposit in South Washixia between 447 ∼ 445 Ma, while the Tamuqie Li pegmatites formed at 448 Ma with Li-barren pegmatites formed at around 418 Ma. It is suggested that the large granitic pegmatite Li deposit in the Central Altun Block, encompassing South Washixia and Tamuqie granitic pegmatite Li deposits, originated during the final collision event of three micro-continent blocks (Qaidam, Altun-Qilian, and Dunhuang-Alex). We suggest that during the final collision and amalgamation event of multiple micro-continents triggering intense compression and thickening of the crust, and the thickened lower crust may undergo shear-driven dehydration melting of biotite in granulite facies to produce large-volume granitic magmas. These magmas crystallize and differentiate into lithium-rich pegmatitic magmas along shear zones, forming large lithium pegmatite deposits. Our research presents a novel mechanism for the formation of granitic pegmatite Li deposits, which emphasizes the role of shear-driven dehydration melting during the final collision and amalgamation events of three micro-continents.

The Permian Cornubian granite batholith, SW England; Part 1: Field, structural, and petrological constraints

The Permian Cornubian granite batholith (295−275 Ma) in SW England includes seven major plutons and numerous smaller stocks extending for ∼250 km from the Isles of Scilly in the WSW to Dartmoor in the ENE. The granites are peraluminous and classified as crustal melt S-type, predominantly two-mica granites, and biotite or tourmaline monzo- and syenogranites, with subordinate minor topaz granite and lithium mica granite. The granites and their host rocks are pervasively mineralized with tin (cassiterite), tungsten (wolframite, ferberite), copper (chalcopyrite, chalcocite, bornite), arsenic (arsenopyrite), and zinc (sphalerite) mineralized lodes. Quartz-muscovite selvedges (greisen-bordered) also contain enrichment of lithophile elements such as boron (tourmaline), fluorine (fluorite), and lithium (lithium-micas such as lepidolite and zinnwaldite). They are derived from both muscovite and biotite dehydration melting of pelitic-psammitic rocks and intruded from a common source along the length of the batholith. Pressure estimates from andalusite and cordierite-bearing hornfels in the contact metamorphic aureole (150 ± 100 MPa) show that the granites intruded to 3 km depth. Cupolas around the Land’s End and Tregonning granites show aplite-pegmatite dikes and tourmaline + quartz + muscovite veins (greisen) that are frequently mineralized. Synchronous intrusions of lamprophyre dikes suggest an additional heat source for crustal melting may have been from underplating of alkaline magmas. The lack of significant erosion means that the source region is not exposed. In an accompanying paper (Part 2; Watts et al., 2024), gravity modeling reveals possible solutions for the shape and depth of the granite and the structure of the lower crust. We present a new model for the Land’s End, Tregonning, and Carnmenellis granites showing a mid-crustal source composed of amphibolite facies migmatites bounded by prominent seismic reflectors, with upward expanding dikes feeding inter-connected granite laccoliths that show inflated cupolas with shallow contact metamorphism. The Cornubian granites intruded >90 m.y. after obduction of the Lizard ophiolite complex, and after Upper Devonian−Carboniferous Variscan compressional, and later extensional, deformation of the surrounding Devonian country rocks. Comparisons are made between the Cornubian batholith and the Patagonian batholith in Chile, the Himalayan leucogranites, and the Baltoro granite batholith along the Karakoram range in northern Pakistan.

Early Cretaceous continental arc magmatism in the Wakhan Corridor, South Pamir: Mantle evolution and geodynamic processes during flat subduction of the Neo-Tethyan oceanic slab

The petrogenesis of continental arc magmas provides critical insights into thermal evolution and geodynamics of the continental lithosphere, crust-mantle interaction, and deep dynamic processes. In this study, we report new zircon U-Pb ages along with isotopic and elemental whole-rock geochemistry, mineral chemistry, and Hf-O isotope data for the Kalaqigu diorites and monzogranites of the Chinese Wakhan Corridor, South Pamir. Zircon U-Pb dating indicates that the Kalaqigu pluton was emplaced in the Early Cretaceous (ca. 108−106 Ma). The diorites are geochemically characterized by low SiO2 (51.9−54.5 wt%) and CaO (7.7−9.4 wt%) contents, but high MgO (5.3−8.3 wt%), Al2O3 (12.8−16.8 wt%), and TiO2 (0.6−1.1 wt%) contents as well as high Mg# (56−65) values. Thus, they are similar to high-Mg diorites: enriched in large ion lithophile elements (e.g., K, Sr, and Ba) and light rare earth elements, while depleted in high field strength elements (i.e., Nb, Ta, Zr, and Hf). Combined with negative εNd(t) (−6.9 to −14.0) and εHf(t) (−9.9 to −12.2), and high (87Sr/86Sr)i (0.7075−0.7086) ratios, these observations indicate that they originated from an enriched lithospheric mantle source. High δ18Ozrn (7.49‰−9.01‰) values, in conjunction with relatively high 207Pb/206Pb and 208Pb/206Pb ratios, suggest that the source was modified by subducted sediment-derived melts. Variable Cr contents (54−117 ppm) are likely controlled by minor fractionation of olivine and orthopyroxene. The monzogranites show high SiO2 contents (69.2−72.0 wt%), and low Rb/Sr (0.4−0.6), (K2O + Na2O)/CaO (2.6−4.8), and FeOT/MgO ratios (2.6−3.2). They contain diagnostic cordierite and show strongly peraluminous characteristics (A/CNK > 1.1) with high δ18Ozrn (7.82‰−8.85‰) values that are compatible with those of typical S-type granites. Their abundant inherited zircons, with age populations similar to those of detrital zircons from regional early Paleozoic metasedimentary rocks, indicate that they were derived from partial melting of ancient metasedimentary rocks. Phase equilibrium modeling is consistent with biotite-dehydration melting of metagreywacke, probably at ∼750 °C and ∼6.0 kbar, as indicated by the biotite chemistry. Based on regional geochronology, a south-to-north magmatic migration suggests that northward flat-slab subduction of the Neo-Tethyan oceanic slab played an important role in the generation of these widespread Early Cretaceous continental arc magmatic rocks. However, the granitoids were generated earlier than the mantle-derived mafic rocks, which suggests that crustal melting occurred during the early stage of subduction. The continuous flat-subduction resulted in partial melting of subducted sediments, which metasomatized the mantle wedge. Contemporaneous regional compression primarily occurred far north of the subduction zone (i.e., North and Central Pamir), inducing deformation as well as crustal shortening. With the flare-up of continental arc magmatism in South Pamir, crustal shortening moved southward. These processes, combined with the addition of voluminous, mantle-derived magmas, played an important role in crustal thickening in Pamir during the Early Cretaceous.

Comparison of Sn-related granitoids in subduction and collision settings by accessory mineral geochemistry: A case study in the Tengchong-Lianghe tin belt, SW China

Magmatic-hydrothermal Sn deposits can develop in both subduction and collision settings. However, a systematic comparison of the geochemical fingerprints and petrogenetic evolution of Sn-related granitoids from both settings is still lacking. The Tengchong-Lianghe tin belt hosts numerous Sn deposits but of different metallogenic ages, such as the Xiaolonghe and Lailishan Sn deposits, which provide an ideal study area for addressing this issue. Zircon U–Pb dating suggests that the Xiaolonghe and Lailishan Sn-related granitoids were emplaced at 74.2–75.5 and 50.0–51.1 Ma, respectively. Given the initial India-Asia collision commenced at ca. 65–60 Ma, these ages, consistent with available Sn metallogenic ages, suggest the formation of the Xiaolonghe and Lailishan Sn deposits in subduction and collisional settings, respectively. New zircon and apatite compositions, and apatite Sr isotopic data, combined with previously reported whole-rock geochemical results, all indicate that the Xiaolonghe and Lailishan Sn-related granitoids share similar magma sources, evolution processes, redox states, and volatile contents. The presence of hornblende, high apatite NdN/NdN* (mostly > 1), and whole-rock evolutionary trends, together indicate that they represent fractionated I-type granites. Distinctly elevated apatite 87Sr/86Sr ratios (0.70988–0.71430) suggest that they originated from an ancient lower crust. Subsequently, they underwent an extensive fractional crystallization dominated by feldspar, as revealed by whole-rock, apatite, and zircon elemental variation trends. The low zircon CeN/CeN* (mainly < 200) and ΔFMQ (–1.9 to 1.7), together with elevated apatite F content (2.30–3.69 wt%), reveal that they crystallized from reduced and F-rich magmas. The relatively high whole-rock zircon saturation temperatures (mainly > 800 ℃) and Ba/Pb ratios imply that they were produced at a higher temperature by biotite-dehydration melting, which requires additional heat from the mantle. The specific mechanism that triggers mantle upwelling (oceanic slab subduction or break-off) could be the most significant difference between Sn-related granitoids formed during subduction and collision processes.

Discovery of the ∼35 Ma granite porphyry dikes in the giant Dachang Sn ore field: Tectonic and metallogenic implications

The world’s second-largest Dachang Sn ore field in the southwest of China exhibits genetic connections to the Late Cretaceous highly evolved S-type granitoids (93–91 Ma). In this study, we report the newly identified Eocene granite porphyry dikes with zircon U-Pb ages of 35–33 Ma at the Gaofeng Sn deposit from the Dachang ore field. The petrogenesis and mineralization potential are also constrained by employing whole-rock and zircon geochemical data. These Eocene dikes coincide with the emplacements of granitoids (35–30 Ma) along the Ailaoshan–Red River fault and its neighboring regions, indicating that they could be related to a similar lithospheric extension setting after the Indo-Asian collision. However, unlike those of the latter granitoids (35–30 Ma), which are grouped into I-type granites, the high-silica granite porphyry dikes show elevated ASI values (1.32–1.38) with high P2O5 (0.41–0.44 wt%) and normative-CIPW corundum (4.15–4.71 wt%) contents and could be classified as S-type granites. The strongly negative whole-rock Sr, Ba, and Eu anomalies signify separation of plagioclase and K-feldspar during magma crystallization, which is also confirmed by the zircon trace elements compositions. The Gaofeng Eocene granite porphyry dikes have a high initial zircon Tti-zr of 855 ℃, high Ba/Pb ratios (11.5–16.8), and low Pb contents (7.56–9.98 ppm) with low zircon εHf(t) values (−7.4 to −0.6), hinting at the dehydration melting of biotite from the metasedimentary basement. Additionally, the dikes are characterized by low oxygen fugacity (zircon Ce/Ce* <70; ΔFMQ from −6.0 to +0.6 with an average of −2.0) and show evidence of magmatic-hydrothermal interaction (whole-rock Nb/Ta < 5), in line with those of the Dachang Late Cretaceous fertile granites, suggestive of Sn mineralization potential. Therefore, it is recommended that attention should be paid to the Eocene granite porphyry dikes during future prospecting.

Mantle contributions to granitoids associated with Sn mineralization: Geochemical and isotopic evidence from the giant Dachang deposit, South China

Major Sn deposits are commonly linked to crust-derived and highly evolved granites, with magma generation aided by mantle heating. However, whether and how the mantle components contribute to Sn polymetallic mineralization remains unclear. In this study, in combination with a compilation of equivalent data in the region, we provide new constraints on this issue based on detailed investigations on the petrogenesis and metallogenic significance of granitoids including the causative batholith and later granodiorite porphyry dike in the giant Dachang Sn deposit from South China. The former has zircon U-Pb ages of 93–91 Ma and belongs to highly evolved S-type biotite granite, which experienced fractionation of massive feldspar. The latter shows zircon U-Pb ages of 90 Ma and displays I-type granite features. The batholith was mainly derived from the dehydration melting of biotite in the metasedimentary sources, as revealed by the relatively low whole-rock Pb contents (<30 ppm) and high Ba/Pb ratios (2.71–17.1) and initial T(ti-zr) of 790 ℃. Compared with the adjacent crust-derived S-type granites (−24.8 – −5.1) and I-type granites (−11.0 to −5.2), the Dachang S-type biotite granites present higher zircon εHf(t) values (−9.1 to −2.1). Furthermore, the low magmatic zircon δ18O values (6.2 ‰) and higher apatite LREE (3277–4114 ppm) and Sr (1137–1357 ppm) contents than of arc-type basic rocks were discerned. These characteristics jointly hint the contributions of mantle components. The higher initial T(ti-zr) (>850 °C), whole-rock Mg# (52 to 58), apatite εNd(t) (−9.2 to −6.5) and zircon εHf(t) (−7.6 to 2.5) values but lower zircon δ18O values (6.33 to 8.30 ‰) of the later granodiorite porphyry dike than those of the batholith also suggest that mantle material was involved in the generation of the dikes, which is evident by the variational features of zircon and apatite trace elements. In addition, at the zircon Hf <12000 ppm and Eu/Eu* > 0.05, the higher zircon ΔFMQ values (mostly from −1.8 to 2.0) and H2O contents (100–1100 ppm) of the Dachang granitoids than the pure crust-derived S-type granites (ΔFMQ = mostly from −3.7 to −1.5; H2O < 100 ppm) imply that mantle materials involved are relatively rich in water and oxidized. These suggest that the addition of mantle components is conducive to the extraction of Sn from metasedimentary sources, and moderately facilitates the increase of oxygen fugacity which still maintains the incompatibility of Sn in magmas with ΔFMQ < 2. Also, the involvement of mantle components upgrades the H2O contents in S-type magmas, favoring the migration of ore-forming elements from magmas to hydrothermal fluids. The sediment-derived causative granites displayed higher εHf(t) and εNd(t) values with greater Sn tonnages of their associated world-class Sn polymetallic deposits, supporting the opinion that the contributions of mantle components play an important role in the generation of giant Sn deposits.

Himalayan-like Crustal Melting and Differentiation in the Southern North American Cordilleran Anatectic Belt during the Laramide Orogeny: Coyote Mountains, Arizona

Abstract The southern US and northern Mexican Cordillera experienced crustal melting during the Laramide orogeny (c. 80–40 Ma). The metamorphic sources of melt are not exposed at the surface; however, anatectic granites are present throughout the region, providing an opportunity to investigate the metamorphic processes associated with this orogeny. A detailed geochemical and petrochronological analysis of the Pan Tak Granite from the Coyote Mountains core complex in southern Arizona suggests that prograde metamorphism, melting, and melt crystallization occurred here from 62 to 42 Ma. Ti-in-zircon temperatures (TTi-zr) correlate with changes in zircon rare earth elements (REE) concentrations, and indicate prograde heating, mineral breakdown, and melt generation took place from 62 to 53 Ma. TTi-zr increases from ~650 to 850 °C during this interval. A prominent gap in zircon ages is observed from 53 to 51 Ma and is interpreted to reflect the timing of peak metamorphism and melting, which caused zircon dissolution. The age gap is an inflection point in several geochemical-temporal trends that suggest crystallization and cooling dominated afterward, from 51 to 42 Ma. Supporting this interpretation is an increase in zircon U/Th and Hf, a decrease in TTi-zr, increasing zircon (Dy/Yb)n, and textural evidence for coupled dissolution–reprecipitation processes that resulted in zircon (re)crystallization. In addition, whole rock REE, large ion lithophile elements, and major elements suggest that the Pan Tak Granite experienced advanced fractional crystallization during this time. High-silica, muscovite± garnet leucogranite dikes that crosscut two-mica granite represent more evolved residual melt compositions. The Pan Tak Granite was formed by fluid-deficient melting and biotite dehydration melting of meta-igneous protoliths, including Jurassic arc rocks and the Proterozoic Oracle Granite. The most likely causes of melting are interpreted to be a combination of (1) radiogenic heating and relaxation of isotherms associated with crustal thickening under a plateau environment, (2) heat and fluid transfer related to the Laramide continental arc, and (3) shear and viscous heating related to the deformation of the deep lithosphere. The characteristics and petrologic processes that created the Pan Tak Granite are strikingly similar to intrusive suites in the Himalayan leucogranite belt and further support the association between the North American Cordilleran anatectic belt and a major orogenic and thermal event during the Laramide orogeny.

Evolution of layering in a migmatite sample: Implications for the petrogenesis of multidomain monazite and zircon

Abstract The timing of partial melting in high-grade metamorphic rocks is critical for constraining tectonic histories and processes. However, uncertainties exist about the behavior of monazite and zircon during partial melting, especially about the timing of crystallization with respect to melting reactions. This study is focused on a single sample (16TG143) of finely layered, migmatitic gneiss from the Adirondack Highlands, New York, interpreted to have undergone extensive biotite dehydration melting (i.e., Bt + Pl + Als + Qz = Grt + Kfs + melt). The rock contains one distinct leucosome layer. The non-leucosome (gray gneiss) portion of the migmatite has millimeter-scale sublayers with distinct differences in modes and mineralogy. The layers are interpreted to reflect the differential preservation of reactants and products formed during the forward and reverse progress of the melting reaction. Monazite and zircon modes, and to some degree, texture, composition, and geochronology all vary from layer to layer. Both minerals have up to three domains: ca. 1150 Ma anhedral cores, ca. 1050 Ma monazite mantles/fir tree textured zircon, and ca. 1030 Ma rims. The heterogeneous layered gray gneiss provides robust constraints on the timing of melting (ca. 1050 Ottawan orogenesis), melt crystallization, and post-melting retrogression, in addition to information about earlier metamorphic events. Early-formed monazite and zircon grains were largely dissolved during progressive melting, except where preserved as relicts or inclusions. Monazite mantles and fir tree zircon grains precipitated upon cooling during progressive melt crystallization between temperatures of 800 and 750 °C. Rims are interpreted to have precipitated during subsolidus, solid-state retrogression after ca. 1050 Ma. Correlations between the gneissic layering, melting reactions, and the character of geochronometers emphasize the importance of characterizing the layer-forming and chronometer petrogenesis processes as a critical part of deconvoluting the history of migmatitic gneisses.

Ultrahigh-Temperature Anatexis of Greywackic Granulites Generates Peraluminous Charnockites in the Jining Complex, North China Craton

Abstract Charnockites form an important component of the lower continental crust. Quantitative investigation on the properties of magmas that can stabilize orthopyroxene at solidi is crucial to understanding the petrogenesis of igneous charnockites. This study has performed detailed petrologic analyses and thermodynamic constraints on the Paleoproterozoic high-maficity (FeOT + MgO = 5–14 wt%) and peraluminous charnockites from the Jining Complex in the North China Craton. These charnockites occur as intrusions in granulite facies terranes and contain the mineral assemblages including prevalent perthitic/antiperthitic feldspars and garnet with minor biotite (usually less than ~5 vol%) beside orthopyroxene. The Jining charnockitic magmas were ascertained to have ultrahigh temperatures up to 1050–1100°C, poor H2O contents around 0.14–0.42 wt% and limited aluminum saturation indexes (ASIs) of 1.0–1.3. The stabilization of orthopyroxene at the solidus is attributed to low magmatic H2O contents and ASIs, which have maxima of 1.2 wt% and 1.5, respectively, and are positively correlated to bulk-rock maficity. Such charnockitic magmas could not release H2O-rich fluids near solidi, as the H2O is buffered by the orthopyroxene–biotite pair. Moreover, combined geochemical discrimination and progressive melting modelling reveal that the Jining charnockites were generated by partial melting of a greywackic granulite source, with about 15%–40% entrainment of solid phases in mushy magmas. The melting occurs at temperatures as high as 1050–1100°C, obviously beyond biotite stabilities, and involves quartz, feldspars and garnet as melting reactants, which differs from the previous proposition that peraluminous charnockites are related to biotite dehydration melting. The resultant magmas are substantially enriched in maficity and depleted in H2O due to both the melt compositions per se and the high entraining capability. Such peraluminous charnockite plutons massively emplaced in granulite facies terranes indicate post-orogenic ultrahigh-temperature anatexis of metasedimentary rocks in conditions close to the crustal dry solidus.

The origin and compositions of melt inclusions in an Al2SiO5‐free paragneiss from the Namche Barwa Complex in the Eastern Himalayan Syntaxis

AbstractMelt inclusions (MIs) in high‐temperature metamorphic rocks provide a unique window into crustal anatexis in collisional orogenic belts and have been widely used to characterize compositions of anatectic melts as well as melting mechanisms. In this study, MIs hosted by peritectic garnet were for the first time identified in an Al2SiO5‐free graywacke‐type paragneiss from the Namche Barwa Complex, the Eastern Himalaya, Southeast Tibet. These MIs occur as nanogranites in the rims of porphyroblastic garnet, exhibit negative crystal shapes with an average diameter of ~12 μm and consist of a mineral assemblage of biotite + quartz + plagioclase + K‐feldspar ± muscovite. Re‐homogenization experiments of these nanogranites were conducted at a pressure of 1.5 GPa and temperatures of 800°C, 850°C and 900°C and produced homogeneous glasses at 850°C. The homogenized glasses are strongly peraluminous and calc‐alkalic in composition, with 66.43–71.31 wt.% SiO2, 12.64–15.06 wt.% Al2O3, high alkaline (5.41–7.22 wt.%) and low ferromagnesian (2.72–4.46 wt.%) contents. They are lower in silica and CaO but higher in K2O compared with MI produced by fluid‐present melting of metasedimentary rocks, thus indicating fluid‐absent melting. These glasses are also characterized by enrichment of large ion lithophile elements (particularly Cs and Rb), depletion of Ba and Sr, low contents of light rare earth elements (3.6 to 33.7 ppm), high Rb/Sr ratios (6.19–37.3) and low Nb/Ta ratios (2.55–18.7). In combination with phase equilibrium modelling, these compositional features suggest that a sequential dehydration melting of muscovite and biotite was responsible for the production of MI during prograde metamorphism of the studied paragneiss. By compiling MI data published in the literature, we show that dehydration melting of metasedimentary rocks from the Himalayan orogen can produce initial melts with various peraluminous and granitic compositions.

Crustal Anatexis and Initiation of the Continental‐Scale Chongshan Strike‐Slip Shear Zone on the Southeastern Tibetan Plateau

AbstractContinental‐scale strike‐slip shear zones often record significant tectono‐magmatism and dynamic deformation processes of the crustal lithosphere. However, the genetic relationships and timing among the anatexis, deformation, and initial shearing along a strike‐slip shear zone have not been well defined. Here, we carried out detailed field, microstructural, zircon U–Pb geochronology, geochemical and EBSD texture analyses of leucogranites and migmatites in the Chongshan shear zone (CS‐SZ). The results indicate that most migmatites and leucogranites exhibit strong shear deformation and well‐developed high‐temperature mylonitic microstructures. The quartz aggregated from foliated leucogranites developed dominant high‐temperature prism &lt;c&gt; and prism &lt;a&gt; slip systems. The pre‐ and syn‐kinematic crustal anatexis and localized weak zone mainly occurred from 35 to 29 Ma along the CS‐SZ, which is closely related to the post‐collisional extension and collapse of overthickened crust. Biotite dehydration melting formed pre‐ and syn‐kinematic melts and leucogranites that further experienced fractional crystallization of plagioclase and K‐feldspar during melting and subsequent melt migration and emplacement upward along the tectonic weak zone. The thinning and weakening of lithospheric crust further facilitated the initial and formation strike‐slip displacement along the CS‐SZ, which occurred from 29 to 20 Ma or much later to 18 Ma. Finally, we propose that crustal anatexis and upward migrating melts play a key role in controlling the thermal state and rheological strength of the crust, resulting in nucleation and initiation of the localized deep‐seated shear zone that accommodates significant displacement for the India–Asia continental forward collision and intracontinental deformation.

Lamprophyre magmatism triggering the formation of the Zhuxi granites related to the world-largest scheelite skarn deposit in South China

The Zhuxi deposit (3.44 Mt. WO3 @0.54%) is the largest tungsten deposit in the world. Although ore-related granitic magmas are correlated with the partial melting of fertile supracrustal material, the heat source triggering the partial melting of fertile supracrustal material has been poorly constrained. To unravel the petrogenesis of the granites, we study the widespread lamprophyre dike swarms coeval with the ore-related granites. Phenocrysts within the lamprophyres are dominated by amphibole, plus a small amount of eastonite. The amphibole phenocrysts exhibit complex internal structures, but the similar geochemical characteristics reveal that they are “antecrysts” recycled from earlier stages of the same magma system that underwent the repeated injection of mafic magmas. Pressure estimates using the Al-in-amphibole geobarometer reveal that most amphibole crystals crystallized at depths of ∼22 km and/or ∼ 10 km, suggesting that lamprophyric magmas most likely ponded at depths of ∼22 km (i.e., the Conrad discontinuity) to form magmatic reservoirs, which in turn fed higher-level magmatic reservoirs that formed at depths of ∼10 km. Both calculations based on energy conservation and simulation results obtained from the quantitative energy-constrained assimilation fractional crystallization (EC-AFC) model consistently reveal that intense partial melting of metagraywackes above the Conrad discontinuity could occur in the temperature interval between 800 °C and 820 °C due to the underplating of lamprophyric magmas. In combination with previous studies showing that the ore-related granitic magmas of the Zhuxi deposit formed at depths of ∼22 km (∼6 kbar) through the biotite dehydration melting of fertile metagraywackes, it can be inferred that the underplating of lamprophyric magmas could have induced the effective partial melting of the overlying fertile supracrustal material, thus forming granitic magmas with significant W endowment. Therefore, the initial appearance of lamprophyres in a district with W-enriched metasedimentary basement rocks should serve as an important prospecting criterion for large-scale tungsten deposits.