Microgranular Enclaves Research Articles

Late Paleozoic slab rollback events in the southeastern part of the Central Asian orogenic belt: Implications for Paleo-Asian Ocean subduction and continental crust growth

The Central Asian orogenic belt is considered to be the largest Phanerozoic accretive orogenic belt on Earth. The late Paleozoic magmatic rocks in central Inner Mongolia are crucial for understanding continental crust growth and the tectonic evolution of the southeastern part of the Central Asian orogenic belt. We present comprehensive geochemical, isotopic, and geochronological data from three late Paleozoic magmatic units in the Mandula area, west of the Solonker suture zone. Zircon U-Pb dating indicates that these rocks formed during the late Carboniferous (316−304 Ma). The Mandula high-Mg diorites exhibit high MgO (3.9−6.5 wt%), high Mg# (61−69), and depleted Nd-Hf isotopic compositions, generated through interaction between a metasomatized mantle and slab melts with the overlying sediments. The Mandula granodiorites display adakite geochemical characteristics with high Sr/Y mass ratios (29−52), high MgO (1.7−2.2 wt%), and high Mg# (52−54), formed by partial melting of the oceanic slab with the addition of overlying sediment. Mafic microgranular enclaves have consistent ages, Sr-Nd-Hf isotope compositions, and hornblende crystallization temperature-pressure conditions with their host granodiorite, formed from a cognate magma associated with the host granodiorites through cumulate. We propose that two phases of slab rollback took place during the late Paleozoic southward subduction-accretion of the Paleo-Asian Ocean. The first phase corresponded to the transformation of low- to medium-angle slab subduction, while the second phase led to subduction-related extension. Considering the tectonic-magmatic evolution, crustal maturity, and thickness variations in the late Paleozoic southeastern part of the Central Asian orogenic belt, we propose that prolonged subduction and slab rollback promoted continental crust growth. The Central Asian orogenic belt coincides temporally and spatially with the Phanerozoic Pangea cycle, suggesting that continuous subduction and supercontinent amalgamation significantly contributed to continental crust growth.

Petrogenesis of episodic volcanic-intrusive rocks in SE China: Crystal-melt segregation and magma mixing

Crystal-melt segregation and magma mixing are key magmatic processes that generate compositional variabilities in igneous rocks. Distinguishing these processes can provide insight into the evolution of volcanic calderas and the driving force of plate subduction. Intensive episodic magmatism occurred in Southeast China during the Cretaceous, producing a series of volcanic-intrusive rocks during different magmatic stages in chain-like calderas. We conducted comprehensive petrological, geochemical, isotopic, and zircon trace element analyses on two representative calderas (Xiaoxiong and Shiniushan).The results show that the magmatism produced multistage volcanic and coexisting intrusive rocks regarding their crystallization ages (102–88 Ma) and NdHf isotopic compositions (zircon ɛHf(t) = −11.7 to −1.8, whole-rock εNd(t) = −8.3 to −2.4). The volcanic and intrusive rocks are related by fractional crystallization.In general, the erupted silicic rhyolitic rocks are characterized by distinctly negative Eu anomalies (δEu = 0.10–0.71) and depletions of Ba, Sr, P, and Ti, while the coexisting intrusive rocks show distinct or complementary geochemical signatures, such as neglectable Eu anomalies (δEu = 0.92–1.09) and positive Ba anomalies. Furthermore, the whole-rock samples from the calderas have variable δ41K values (from −0.14 ‰ to −0.59 ‰) that show strong correlations with indices of fractional crystallization such as Eu anomaly, Sr content, and K2O/Na2O ratios, indicating a key role of plagioclase during crystal-melt segregation. This is also supported by petrographic evidence for crystal accumulation such as the synneusis of plagioclase phenocrysts. Mafic microgranular enclaves in the volcanic-intrusive rocks indicate that magma recharge events occurred in the magma reservoir. Synthesizing these lines of evidence, we argue the multiple magmatism in plate subduction zone can be sustained by recharge events in the magma reservoir along with crystal-melt segregation processes. Particularly, the volcanic rocks tend to have isotopically lighter K than intrusive rocks, and strongly support that the volcanic rocks represent the extracted melt that is complementary to the cumulative intrusive rocks.

Petrogenesis of early Paleozoic syn-collisional granitoids and enclaves in Western Kunlun, Northwest China: Implications for the growth of continental crust

The Western Kunlun Orogenic Belt (WKOB), along the northwestern margin of the Qinghai–Xizang Plateau, was formed by the collision of Gondwana-derived terranes to the south and the Tarim Block to the north and was closely associated with closure of the Proto-Tethys Ocean during the late Neoproterozoic to early Paleozoic. We present a combined zircon UPb geochronology, whole-rock composition, and Sr–Nd–Hf isotopic study of syn-collisional granitoid plutons and mafic microgranular enclaves (MMEs) in the region. Zircon UPb dating yields ages of 443.8 ± 4.4, 451.9 ± 4.2, 462.9 ± 3.5, and 456 ± 4.2 Ma for the Tongayoupuagezi, Shanjie, and Pishigai plutons and MMEs from the Pishigai pluton, respectively. The granitoids are metaluminous to weakly peraluminous (A/CNK = 1.00–1.17) and belong to the high-K calc-alkaline series. They have (87Sr/86Sr)i ratios of 0.7058–0.7154, εNd(t) values of −8.78 to −0.93, and εHf(t) values of −19.72 to +6.87. The MMEs have variable SiO2 contents (45.7–60.2 wt%) and are more mafic than the host granitoids, but have similar Sr–Nd–Hf isotopic compositions to the host granitoids [(87Sr/86Sr)i = 0.7102–0.7110; εNd(t) = −6.57 to −3.56; εHf(t) = −7.17 to −1.81]. The MMEs are fragments of cumulates formed during the early stages of magma evolution. The granitoids were produced by the partial melting of a mélange source. The new data support the view that the Middle–Late Ordovician syn-collisional granitoids with MMEs distributed along the WKOB represent a magmatic response to terrane collision. This suggests that juvenile crustal growth in older orogenic systems, which occurs by arc addition, also involves some vertical addition during the final stage of orogenic collision. Our study suggests that mélange diaper melting is a key mechanism of crustal growth during the syn-collision stage in continental collision zones, associated with slab breakoff.

Zircon U-Pb-Hf isotopes constraints on the petrogenesis of Neoarchean granitic rocks from the Northeastern part of the Eastern Dharwar Craton, Southern India

In the present study, we integrate bulk rock chemistry and in-situ zircon U-Pb-Hf isotopes of high-K Neoarchean granitoids from the northeastern part of the Eastern Dharwar Craton (EDC) to investigate their source, petrogenesis, and plausible tectonic setting. Based on the mineral assemblage, in coherence with their geochemical characteristics, these granitoids are classified as Hornblende-biotite granite (HBG) with Microgranular enclaves (ME), Biotite granite (BTG) and Monzogranite (MG). Field observations and zircon U-Pb ages reveal coeval emplacement of these granitoids between 2.53 and 2.50 Ga. The HBG, including ME, has low silica, metaluminous affinity, and high ferromagnesian element content, consistent with their derivation from a mafic source. Some of the HBG and ME samples are strongly enriched in incompatible elements, similar to the sanukitoids. The geochemical attributes and strongly evolved zircon Hf isotopic compositions (ɛHf(t) = −4.3 to +1.8 and −7.3 to +0.2) of the HBG and ME suggest their derivation from a mantle source metasomatized by subducted sediments. The BTG and MG are compositionally similar, with high silica, low ferromagnesian element content, and moderate peraluminous affinity, suggesting the involvement of felsic crustal sources. The sub-chondritic to chondritic zircon Hf isotopic compositions (ɛHf(t) = −2.7 to +1.4 and −6.9 to +1.9) of these granitoids support the involvement of heterogeneous ancient crustal sources. While all granitoids exhibit similar zircon Hf isotopic compositions, their geochemical attributes suggest distinct sources. The HBG reflects juvenile crustal additions, whereas BTG and MG are products of the crustal reworking. We propose that these granitoids formed during the continent-continent collision between Western and Eastern Dharwar cratons, that took place after the break-off of the eastward subducting slab. The asthenospheric upwelling induced by slab break-off and/or lithospheric delamination of thickened crust led to the genesis of metasomatized mantle derived magma, which drove the crustal reworking. The observed zircon Hf isotopic composition of the EDC granitoids are similar to the present day Phanerozoic Alpine-Himalayan type orogenies supporting subduction followed by continental collision model for the stabilisation of Archean cratons. Evidence for the existence of Paleo- Mesoarchean felsic crust from this part of the EDC warrants further studies to test the three-terrane model for the Dharwar Craton.

Petrogenesis of microgranular enclaves in the A-type granitoid Krasnopol intrusion (Mazury Complex, northeastern Poland): Evidence of magma mixing

The origin of magmatic microgranular enclaves has been investigated in the Mesoproterozoic granitoid Krasnopol intrusion (1.5 Ga), part of the AMCG (anorthosite–mangerite–charnockite–granite) Mazury Complex in the East European Craton (NE Poland). The granitoids are ferroan and metaluminous, and display the typical characteristics of A-type granites, with high contents of Zr, Nb, Ga and rare earth elements (REEs). The enclaves are metaluminous and have a broad compositional range with two groups distinguished: silica-poor (45–50 wt% SiO2) and silica-rich (54 to 59 wt% SiO2), the latter overlapping in composition with the granitoid samples. The silica-poor enclaves are enriched in REEs compared to the silica-rich type, while the silica-rich enclaves exhibit trace-element patterns similar to those of the granitoids. Initial whole rock εNd values range between -3.8 and -4.0 for the granitoids and give a slightly wider range from -2.6 to -3.8 for the enclaves. The 87Sr/86Sr initial values vary from 0.7084 to 0.7138 for the granitoids and between 0.7052 and 0.7075 for the enclaves and indicate that the granitoids and enclaves are not isotopically identical. These may suggest that the two magmatic systems represented by the granitoid host rock and the enclaves, were probably derived from different sources, but with sufficient interaction, which led to a progressive change in the composition of the enclaves towards intermediate composition. We suggest that the mafic melts of the enclaves were generated at the base of the thickened crust through partial melting of the lower crustal source, with a significant contribution from mantle material. The increase in temperature resulted in anatexis of the lower crust and the formation of the granitoid parental magma.

Magmatic evolution and sources of metals and fluids in the large granite-related Xiasai vein-type Ag–Pb–Zn(–Sn) deposit, northern Yidun terrane, SW China

The Xiasai Ag–Pb–Zn–(Sn) deposit in the northern Yidun terrane (eastern Tethys), hosted in Late Triassic metasandstones, is genetically associated with Late Cretaceous granites. We present chemical and Sr–Nd–Pb–Li–B isotope data of Late Cretaceous granites and their mafic microgranular enclaves (MME), late aplites, and Late Triassic metasedimentary rocks. The narrow range of 87Sr/86Sr99 (0.70627–0.70953) and εNd99 (–6.1 to –5.2) values of most Late Cretaceous granites reflects a mixed source with contributions from the mantle and Paleoproterozoic felsic crust. These granites have high SiO2 contents (>69 wt%) and strong depletions in Sr, Ba, and Eu. Major and trace element contents correlate with the fractionation index 1/TiO2, indicating that they experienced extensive fractionation. Local late aplite dikes were derived from similar source rocks, but seem to be younger.The Pb isotopic compositions reflect that the metal Pb was derived from the granite with variable contributions from the Late Triassic sedimentary rocks. Weakly and moderately altered metasandstones have similar δ7Li and δ11B values as most granites (δ7Li: 0.00 to 2.68 ‰; δ11B values: –14.93 to –12.36 ‰), indicating magmatic fluid sources. The magmatic fluids resulted in the initial enrichment of Sn, Cu, Pb, and Zn. Strong alteration of feldspar and biotite resulted in the metasandstones in negligible change of Li concentrations, significant loss of Sr, Na2O, CaO, MgO, and Fe2O3, but significant addition of B. This alteration lowered δ7Li from 1.01 to –3.65 ‰ and lowered δ11B from –9.43 to –19.18 ‰. The low δ7Li values and extremely low δ11B values reflect external fluids from the Late Triassic sedimentary wall rocks, which interacted with granites and mobilized metals (e.g., Sn, Ag, Pb, and Zn) from the granite and/or the sedimentary wall rocks. The location of the Xiasai deposit is controlled by regional NNW-trending faults induced by Late Cretaceous extension.

The Capability of Amphibole in Tracing the Physicochemical Processes of Magma Mixing

AbstractThis study explores the capability of amphibole in tracing the physicochemical process of magma mixing through spatially associated gabbros, mafic microgranular enclaves (MMEs) and granodiorites from central Tibet. These rocks share similar zircon ages as well as zircon Hf‐O and plagioclase Sr isotopes. However, the amphiboles within the gabbros and granodiorites have different Sr and B isotope compositions, while amphiboles with both heterogeneous isotopic imprints occur in the MMEs. According to data and modeling, significant mixing of two isotopically distinct magmas is recorded by amphibole but not by zircon and plagioclase. Based on a synthesis of petrography, geochemistry and thermobarometry, we interpret this inconsistency by the crystallization order of minerals and propose that magma mixing occurred after the parent magma was emplaced at ∼10 km and cooled to ∼750°C. Our study highlights that amphibole may be a more sensitive tracer of magma mixing relative to other commonly used methods.

Iron isotope constraints on the genesis of giant Beiya Au-base metal deposits, Yunnan, Southwest China

The Beiya Au-base metal deposit in southwest China is characterised by a huge amount of iron associated with gold mineralization. The formation of the Beiya deposit is generally thought to be related to Tertiary potassic intrusion, as porphyry- and skarn-type mineralization are the two most important types of mineralization in the region. However, the lack of direct evidence to constrain the source of iron and gold as well as other metals makes it difficult to accurately conceptualise the ore genesis model. In this study, we report high-precision (0.03 ‰; 2sd) Fe isotope data on Fe-bearing minerals including magnetite, pyrite and chalcopyrite, and major igneous rocks exposed in the area, including monzogranite porphyry (MGP), mafic microgranular enclave (MME), lamprophyre and basalt. Magnetites have a wide range of δ Fe from 0.15 to 0.64 ‰, reflecting substantial isotopic fractionation at different mineralization stages. MMEs have heaviest Fe isotopic composition (0.35 ‰ to 0.88 ‰) compared to MGPs (0.19 ‰ to 0.48 ‰), lamprophyre (∼0.2 ‰) and basalt (∼0.0 ‰). A negative correlation between δ56Fe and TFe2O3 is found for both MPGs and MMEs, which indicates the leaching of iron by hydrothermal fluids mobilizes light Fe and leaves behind isotopically heavy Fe in the remaining source rock. Fe isotopic compositions of MPG and MME overlap with those of magnetite, both of which are proposed to be the source of Fe in magnetite. The basalt has δ56Fe value of 0 ± 0.03 ‰, similar to those of global basalts, suggesting a negligible contribution to the formation of iron ores. Therefore, we propose that magmas parental to the MGP and MME may be the main source of iron to form the Beiya deposit; post-magmatism hydrothermal activities play a key role in leaching the metals out and transporting them to the mineralization area.

Chemical-memory of biotite crystals preserves the temperature and pressure conditions of the mixing process between trachytic and rhyolitic magmas in the Glória Sul stock, Sergipano orogenic System, Northeast Brazil

Biotite is an important constituent in the microgranular syenitic enclaves of the Glória Sul stock, located in the central region of the Sergipano Orogenic System, Northeast Brazil. These enclaves are centimeter-sized, rounded, and ellipsoidal, finer-grained than the surrounding granites. They are composed of microcline, orthoclase, albite-oligoclase, diopside, quartz, hornblende, titanite, apatite, zircon, epidote, magnetite, ilmenite, thorite, and pyrite. The textures present in the enclaves indicate mixing between magmas and rapid cooling of the trachyte magma that formed the microgranular enclaves (fine-grained texture, acicular apatite and zircon, blade-shaped biotite). The studied biotite exhibits a magmatic texture and is chemically classified as Fe- and Mg-biotite, particularly those with TiO2 > 2.3%. The chemical evolution of biotite is controlled by two substitutions (VI R2+ + 2 VI Al3+ ═ VI □ + 2 VI Si4+ and VI R2+ + VI Ti ═ 2 VIAl3+) reflecting a decrease in titanium and an increase in aluminum. The chemical data of biotite crystals indicate that they crystallized from I type magma, with calc-alkaline affinity, suggesting that the rocks of Gloria Sul stock were formed by the interaction between mantle- and crustal-derived magmas. Thermobarometry of magmatic biotite (enclaves and granites) provided crystallization temperatures between 900 and 800 °C and a depth of 36 to 13 km. The interaction between trachytic (enclave) and rhyolitic (granite) magmas in Glória Sul stock at occurred at about 26 km depth and at a temperature about 800 °C.

Geochronology and petrogenesis of multiple magmatic events in the Tianbaoshan orefield, NE China: Implications for tectonic evolution

Recent studies have shown that multistage magmatic and metallogenic events in NE China were dominated by the Paleo-Asian Ocean tectonic regime and the Paleo-Pacific Ocean tectonic regime. However, outstanding questions remain on the petrogenesis and fertility of ore-causative magma in each metallogenic event. In this study, we report new geochronologic and geochemical data on ore-causative intrusions from the Tianbaoshan orefield in the east Jilin-Heilongjiang belt (EJHB), aiming to identify their petrogenesis, magma fertility, and their implications for the geodynamic evolution of the EJHB. Middle Permian Lishan ore-causative quartz monzodiorites with abundant mafic microgranular enclaves (MMEs) were emplaced at ca. 264.9 ± 2.6 Ma. Petrographic and geochemical characteristics (87Sr/86Sr(i) = 0.7049–0.7053, εNd(t) = 2.9–3.7, and εHf(t) = 4.7–11.7) indicate a juvenile crust source injected by a slab-metasomatized mantle component. The Early Jurassic Beishan monzogranites (ca. 192.5 ± 1.8 Ma) were generated by the partial melting of the juvenile underplating basaltic lower crust with subsequent fractional crystallization in response to their highly evolved geochemical features, combined with their depleted Sr-Nd-Hf isotopic signature (ISr = 0.7032–0.7037, εNd(t) = 2.8–3.3, and εHf(t) = 7.2–12.2). Based on the zircon trace element geochemistry, we infer that the Middle Permian quartz monzodiorites had a high oxygen fugacity (Ce4+/Ce3+ = 40–216), potentially generating the Pb-Zn-Cu mineralization. Early Jurassic Beishan monzogranites had a lower magmatic oxygen fugacity (Ce4+/Ce3+ = 14–106), which may account for the Mo-dominated mineralization. A combination of this study and previous results corroborates that the Tianbaoshan orefield records the superposition of different tectonic regimes during metallogenesis.

Petrogenesis of K-rich and high-mg adakitic rocks in the North Dabie terrane, China: products of early cretaceous orogenic collapse

ABSTRACT Understanding orogenic collapse is key to comprehending the evolution of continental collision orogens. The Dabie orogen is a collapsed continental collision orogen that provides an ideal window for revealing orogenic collapse. Here, we conduct fieldwork, whole-rock geochemistry, and zircon U-Pb dating of the Daoshichong pluton in the North Dabie terrane to elucidate the collapse mechanism of the Dabie orogen. The Daoshichong pluton is composed of quartz monzodiorite and mafic microgranular enclaves (MMEs) with an Early Cretaceous (~130 Ma) crystallization age. The Daoshichong pluton is strongly deformed and ductile, displaying a gneissic structure. The quartz monzodiorites are K-rich and high-Mg adakitic rocks. They contain 60.2–63.3 wt.% SiO2 and have high K2O (2.88–3.42 wt.%) and MgO (2.08–3.04 wt.%) contents with relatively high Mg# values (44.4–49.3). They exhibit high Sr (673–819 ppm) and low Y (11.4–20.9 ppm) and heavy rare earth elements (e.g. Yb: 0.94–1.87 ppm) contents, resulting in high Sr/Y (35.1–72.0) and (La/Yb)N (15.7–36.1) ratios. The MMEs exhibit geochemical characteristics similar to those of the Early Cretaceous mafic – ultramafic rocks of the Dabie orogen. They have low SiO2 contents (48.1–51.9 wt.%) and high MgO contents (3.27–5.62 wt.%) with high Mg# values (47.3–51.2). They also exhibit relatively high Ni (16.6–63.6 ppm), Cr (30.5–164.6 ppm), Y (18.2–25.8 ppm) and Yb (1.50–1.94 ppm) contents and high Sr/Y ratios (53.7–109.5). Evidence from fieldwork and geochemistry distinctly indicates magma mixing during magma evolution between the MMEs and the quartz monzodioritic host. Thus, the Daoshichong quartz monzodioritic host and the MMEs can be interpreted as a mixture of mantle-derived magma and over-thickened lower continental crust-derived adakitic melt. Previous studies suggest that the Early Cretaceous (~130 Ma) high-Mg adakite-like rocks are distributed only along the eastern margin of the Dabie orogen (i.e. along the Tan – Lu Fault). This study is the first to report a homochronous (~130 Ma) K-rich and high-Mg adakite-like pluton in the core of the Dabie orogen. Our results suggest that a whole delamination of the over-thickened orogenic root caused the orogenic collapse, subsequently triggering crust – mantle interactions, dome structure, and magmatism in the Dabie orogen.

Mineral Chemistry, Trace Elements, Isotopic Analysis and Zircon U‐Pb Dating in the Hesar Pluton, Northern UDMA, Iran: Implications for Pre‐Collisional Magma Mixing

AbstractThe Hesar pluton in the northern Urumieh–Dokhtar magmatic arc hosts numerous mafic‐microgranular enclaves (MMEs). Whole rock geochemistry, mineral chemistry, zircon U‐Pb and Sr‐Nd isotopes were measured. It is suggested that the rocks are metaluminous (A/CNK = 1.32–1.45), subduction‐related I‐type calc‐alkaline gabbro to diorite with similar mineral assemblages and geochemical signatures. The host rocks yielded an U‐Pb crystallization age of 37.3 ± 0.4 Ma for gabbro‐diorite. MMEs have relatively low SiO2 contents (52.9–56.6 wt%) and high Mg# (49.8–58.7), probably reflecting a mantle‐derived origin. Chondrite‐ and mantle‐normalized trace element patterns are characterized by LREE and LILE enrichment, HREE and HFSE depletion with slight negative Eu anomalies (Eu/Eu* = 0.86–1.03). The host rocks yield (87Sr/86Sr)i ratios of 0.70492–0.70510, positive εNd(t) values of +1.55–+2.06 and TDM2 of 707–736 Ma, which is consistent with the associated mafic microgranular enclaves ((87Sr/86Sr)i = 0.705014, εNd(t) = +1.75, TDM2 = 729 Ma). All data suggest magma‐mixing for enclave and host rock formation, showing a complete equilibration between mixed‐mafic and felsic magmas, followed by rapid diffusion. The TDM1(Nd) and TDM2(Nd) model ages and U‐Pb dating indicate that the host pluton was produced by partial melting of the lower continental crust and subsequent mixing with injected lithospheric mantle‐derived magmas in a pre‐collisional setting of Arabian–Eurasian plates. Clinopyroxene composition indicates a crystallization temperature of ∼1000°C and a depth of ∼9 km.

Mafic microgranular enclaves and magmatic fabrics in the Ediacaran granitoids of Saghro, Eastern Anti-Atlas: Insights into magma emplacement and tectonic evolution

The Saghro inlier in the Anti-Atlas belt of Morocco comprises several granitoids dating from 600 to 550 million years ago (Ma), which are overlain by a volcano-sedimentary series. In particular, the granodioritic plutons exhibit widespread enclaves, which is a tool for emplacements mode and magma interaction processus during the Ediacaran evolution of the Anti-Atlas belt. This paper presents a detailed field mapping, petrographic analysis, and deformation quantification of the mafic microgranular enclaves (MMEs) and their host granitoids to characterize the emplacement mode of the plutons and the origin of MMEs.Macroscopically, the MMEs are rounded to ellipsoidal shapes and characterized by sharp, crenulated contacts, fine-grained textures, and some of them include fragments of host rock and feldspar xenocrysts. Petrographically, the MMEs exhibit a range of compositions from gabbroic to quartz diorite, and they are characterized by the presence of acicular apatite, quartz-feldspar ocelli, poikilitic and zoned plagioclases, which are the result of mixing/mingling of mafic and felsic magmas during the genesis of MMEs. The magmatic structures and enclave's organization in Saghro granitoids indicate two styles of emplacement. The c. 600 Ma Iknioun granodiorite exhibits an S-sigmoidal structure, representative of a syntectonic emplacement model within a transpressive dextral movement along E-W shear zones. While, in the 580-560 Ma Oussilkane, Tagmout, Igoudrane, and Bou Teglimt plutons, the organization of magmatic flow and orientations of enclaves suggest that the magma was emplaced as a post-tectonic dome. The variation in emplacement mode and the decrease in deformation intensity from the c. a. 600 Ma in the Iknioun pluton to the 580-560 Ma for the others plutons indicate a progressive shift in deformation regime from transpressive during the middle Ediacaran to a transtensive regime in the late Ediacaran, highlighting the progressive transition from Pan-African/Cadomian orogeny to Cambrian rifting.

Early Triassic high-K granitoids and enclaves of the Daheba pluton, West Qinling (China): Implications for relative contributions of crust and mantle

Early Triassic granitoids are widespread in the northwestern West Qinling Orogen, China, but their petrogenesis and geodynamic implications remain unclear. In this study, we integrated new field and petrological observations, mineralogical compositions, zircon U-Pb dating, Hf isotopic and whole-rock geochemical analyses for the early Triassic granitic pluton in the Daheba area to determine its magma source and geodynamic scenario. The pluton is composed of granodiorite and syenogranite and carries microgranular enclaves (MEs) which formed at ca. 253–249 Ma. The granitoids show wide SiO2 contents of 63.73–77.49 wt% (av. 69.89), high K2O contents of 3.4–5.4 wt% (av. 4.1) and moderate Mg# of 16–51 (av. 36), belonging to high-K, calc-alkaline I-type granites. These rocks have high radiogenic but uniform Hf isotopic compositions with 176Hf/177Hf and ɛHf(t) of 0.282511–0.282658 and −3.85 – +1.25, respectively. The MEs hosed within the granite are characterized by high Mg# of 35–58 (av. 46) and variable εHf(t) of −4.64 – +9.35, which likely represent a hybridized melt derived from a slab-modified mantle and lower crust. In combination with coeval magmatic rocks in the western Gonghe-East Kunlun area, we propose a genetic model where mafic magmas derived from an enriched mantle and underplated beneath the overlying lower crust are considered to have produced the high-K felsic magma. Further, the hybridized melt ascended to shallower crustal levels to generate a series of rocks ranging from dioritic MEs to granodiorite to syenogranite. Mass balances modeling suggests that the generation of these rocks involved 36 % of the lower crustal-derived melt and 64 % of the SCLM (R2 = 0.9). Our new data, in tandem with published results suggest that the Daheba pluton formed during the subduction stage of the Paleo-Tethyan Ocean and that a local extensional episode occurred at 253–249 Ma in the western Gonghe area.

Syntectonic Shear-induced Emplacement of Crystallizing Granite Magmas Evident from Magmatic Shear Sense, Mafic Schlierens, and Microgranular Enclaves in the Mesoproterozoic A-type Kanigiri Pluton, Nellore Schist Belt, Southeast India

ABSTRACT The observations on field-based mesoscale magmatic structures suggest that the A-type Kanigiri granite (KG) pluton, Nellore Schist Belt (NSB) has undergone a long plutonic history as evidenced by the continuum of deformation from early magmatic to the ductile regime at the waning stage of pluton evolution. The linear alignment of the KG pluton, which lies sub-parallel to the regional Terrane Boundary Shear Zone (TBSZ), and the long-standing deformation regime indicate the genetic link with this shear zone. The formation of mafic schlierens in the KG pluton owes to the mechanical crystal flow-sorting process during the replenishment of KG magma, which is further induced by the injection of crystal-charged microgranular enclave (ME) magmas in the crystallizing felsic magma chamber. The outflown ME magma blobs from the walls of the conduit during the invasion trigger the local turbulence, resulting the swirling of early crystallized mafic crystals of the host KG magma. This kind of swirling in the host KG magma must have been formed at an early stage of crystallization i.e., crystal-poor condition of the KG melts. When the MEs flow out, gravity causes them to interact with the host KG magma and attempt to dissolve into it. This results in schlieren rims formed around the MEs. The magmatic shearing observed in the KG pluton results from an active crystal-mush environment due to accumulated strain caused by the syntectonic movement as evidenced by the magmatic sense of shears. The locally formed slickensides are sub-magmatic origin during the upward pushing of replenishing ME and another pulse of KG magma. The presence, spread and prevalence of magmatic structures like ME magma globules, magma flowage, mafic schlierens, slickensides and shear sense indicators in the KG pluton suggest that the KG pluton underwent dynamic magma emplacement and evolution due to a shear zone, probably the TBSZ, which acted upon KG pluton in a syntectonic environment.

Metallogenic Difference between the Late Aptian Nansu and Aishan Pluton in Jiaodong: Constraints from in Situ Apatite Elemental and Nd Isotopic Composition

A series of Mo-polymetallic deposits have been developed in the Jiaodong Peninsula. Notably, these Mo-dominant deposits formed essentially during the same period as the well-known world-class Au deposits in this area, hinting at a potentially unique geological correlation between them. Therefore, conducting thorough research on Mo deposits in Jiaodong holds significant importance in exploring the area’s controlling factors of Mesozoic metal endowments. To reveal the petrogenesis and metallogenic potentials of Mo-fertile and ore-barren granitoid, apatite grains from the Late Aptian Nansu granodiorite and Aishan monzogranite are investigated in this study. Detailed petrographical observations, combined with in situ analysis of electron probe micro-analyzer (EPMA) and Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), have been conducted on apatite grains from the Nansu and Aishan plutons. This comprehensive analysis, encompassing both major and trace elements as well as isotopic characteristics of apatite, aims to elucidate the metallogenic differences within the late Early Cretaceous granitoids of Jiaodong. The results reveal that the apatite grains across all samples belong to fluorapatites, suggesting their magmatic origin. Additionally, chondrite-normalized rare earth element (REE) patterns of apatites in ore-fertile and ore-barren granitoids exhibit a “right-leaning” trend, characterized by relative enrichments in light REEs and depletions in heavy REEs. Both the Nansu and Aishan plutons exhibit moderately negative Eu anomalies (with averages δEu values of 0.44 and 0.51, respectively), along with slightly positive Ce anomalies (averaging δCe values of 1.08 and 1.11, respectively). A negative correlation is observed between their δEu and δCe values, indicating that the parental magmas of ore-fertile and ore-barren granitoids were formed in a relatively oxidizing environment. The calculated apatite OH contents for the Nansu pluton range from 0.26 to 1.38, while those for the Aishan pluton vary between 0.24 and 1.51, indicating comparable melt H2O abundances. Consequently, the results suggest that neither the oxygen fugacities nor the water contents of the parental magma can account for the metallogenic differences between Nansu and Aishan plutons. The apatite in the Nansu pluton exhibits a higher Ce/Pb ratio and a relatively lower Th/U ratio, indicating the involvement of a greater volume of fluids in the magmatic evolution process of this ore-bearing granitoid. Apatite grains sourced from the Nansu and Aishan plutons exhibit εNd(t) values ranging from −16.63 to −17.61 (t = 115.7 Ma) and −17.86 to −20.86 (t = 116.8 Ma), respectively. These results suggest that their parental magmas primarily originated from the partial melting of Precambrian metamorphic basement rocks within the North China Craton, with a minor contribution from mantle-derived materials. Additionally, the presence of mafic microgranular enclaves in both the Nansu and Aishan plutons indicates that both have undergone magma mixing processes. The binary diagrams plotting the ratios of Ba/Th, Sr/Th, and U/Th against La/Sm demonstrate that apatite grains of ore-fertile granitoid exhibit a distinct trend towards sediment melting. This suggests the potential incorporation of sedimentary materials, particularly those rich in molybdenum, into the magmatic source of the Nansu pluton, ultimately leading to the occurrence of molybdenum mineralization.

Origin of mafic microgranular enclaves from the Late Triassic Cuojiaoma granitic pluton: Implications for the tectonic evolution of the Yidun arc belt, eastern Tibetan Plateau

In-situ U-Pb ages and Hf-O isotopes of zircon are frequently used to study the petrogenesis of mafic microgranular enclaves (MMEs) in granitoids. However, whether zircons in MMEs have recorded the primary isotopes of their host MMEs is still questionable. In this study, MMEs and host granites from the Cuojiaoma granitic pluton in the Triassic Yidun arc belt, eastern Tibetan Plateau, are studied. LA-ICP-MS zircon U-Pb dating of studied MMEs yields an age of 213 ± 2 Ma. Model calculations indicate that the whole-rock composition of the MMEs was different from the melt composition from which the zircon grains grew. The MMEs crystallized during magma mixing based on analyses of zircon trace element and Zr-Hf isotopes. None of zircons in this study have recorded the primary isotope signature of the mafic endmember of MMEs because they are different from the host granite in terms of zircon morphology, ɛHf values and ΔQFM. The Zr isotopic data demonstrate open-system magmatic crystallization for the host granite and that magma mixing for MMEs play key role in controlling Zr isotopic variations. Initial 87Sr/86Sr ratios of An-rich plagioclase crystals in groundmass from MMEs range from 0.7027 to 0.7058, which are significantly lower than host granites (0.7080 – 0.7089). The lowest initial 87Sr/86Sr ratios (0.7027 – 0.7028) are also lower than arc volcanic rocks and similar to a depleted mantle. Combined with previous studies, we propose that MMEs were generated during post-collision extension by mixing of mafic magma of Xiaxiaoliu basalt and felsic magma from host granite.

Origin of granitoids and coeval mafic microgranular enclaves in the East Kunlun Orogen, northern Tibetan Plateau: Insights from petrography, U-Pb geochronology, geochemical and isotope analyses

Granitoids and coeval mafic microgranular enclaves (MMEs) provide important insights into petrogenesis and crustal evolution. In the eastern sector of the East Kunlun Orogen, the Halazha (HLZ) pluton contains abundant MMEs, recording magma mingling processes. The HLZ pluton comprises quartz diorites and granodiorites, whereas the MMEs have a gabbroic-dioritic composition. This paper provides a detailed description of the petrography and mineralogy of the HLZ pluton and MMEs, and presents new ages and geochemical data. The zircon U–Pb dating results indicate that quartz diorites and granodiorites are ranging from 243 ± 2 Ma to 244.8 ± 1.3 Ma and from 233.3 ± 1.1 Ma to 233.6 ± 1.5 Ma, respectively. The zircon ages of the coeval MMEs are identical to the host rocks, i.e., 244.5 ± 1.1 Ma − 244.5 ± 1.2 Ma, and 233.7 ± 1.3 Ma, respectively. Both quartz diorites and granodiorites contain hornblende, display metaluminous characteristics with average A/CNK values of 0.93 and 0.99, respectively, and thus belong to I-type granite. Both of them are enriched in large-ion lithophile elements such as Rb and Ba and depleted in high-field-strength elements such as Nb and Ta. In addition, they both have low εHf(t) values, ranging from −5.6 to −1.7 and −5.4 to −1.79, respectively. Results suggest that HLZ pluton is derived from magmas formed by the partially melting juvenile mafic lower crust. Quartz diorites experienced hornblende accumulation for its low SiO2 contents. Whereas, the high Sr/Y ratio of granodiorites attributes to the fractional crystallization of apatite. Notably, the crystallization pressure and temperature (average 3.43 kbar and 764 °C) of the hornblende xenocrysts in MMEs is close to it of the hornblende phenocryst in quartz diorite (average 3.68 kbar and 766 °C). This similarity indicates that MMEs in the quartz diorites resulted from magmatic mingling between the mafic magma and the host rock within the deep crust. The hornblende xenocrysts present in MMEs of the granodiorites contain a higher number of inclusions, reveal higher crystallization pressures (average 2.73 kbar) and temperatures (average 825 °C) compared to their host rocks (average 1.35 kbar and 702 °C). These results suggest that MMEs in granodiorites underwent two distinct magmatic mixing processes: the magmatic mixing of mafic magma in the deep crust and then during the upwelling process after this mixed mafic magma entered the granodiorites.

Post-collisional magmatism and local lithospheric thinning in the Neoproterozoic Alto Pajeú Domain, northeastern Brazil: Geochemical and isotopic evidence

The 595–560 Ma post-collisional magmatism in the Alto Pajeú Domain, Borborema Province, northeastern Brazil, is characterized by the intrusion of voluminous high-K calc-alkaline, shoshonitic and ultrapotassic magmas. The nature of the source rocks and the geodynamic context associated with the generation and emplacement of this post-collisional magmatism is still enigmatic. In this study, we report new UPb ages, elemental geochemistry, mineral chemistry and SrNd isotope data for rocks from the Pajeú batholith. LA-ICP-MS zircon UPb dating revealed that this batholith was formed by at least two distinct magma pulses. The oldest pulse (ca. 592 Ma) consists mainly of porphyritic quartz monzonites to monzogranites with abundant mafic microgranular enclaves (MMEs), while the younger pulse (ca. 568 Ma) is composed of equigranular biotite granites. The porphyritic monzogranites are magnesian, have intermediate to acidic compositions (SiO2 = 62.2–67.6 wt%) and high Mg# (44–55) values. In contrast, biotite granites are ferroan, have high SiO2 (69.3–73.1 wt%) and low Mg# (16–36) values. Both rock types are characterized by enrichment in LREE and LILE and depletion in HREE and HFSE, but with more fractionated REE patterns for biotite granites ([La/Yb]N = 67.4–101.5) than for porphyritic rocks ([La/Yb]N = 35.8–54.8). The SrNd isotopic values for the different rock types are roughly similar. However, biotite granites show slightly higher initial 87Sr/86Sr (0.71265–0.71412) ratios and more negative εNd (t) (−18.45 to −18.67) values than porphyritic monzogranites (initial 87Sr/86Sr ratios from 0.71077 to 0.71155 and εNd (t) from −16.04 to −16.96). These geochemical and isotopic signatures, together with mineral chemistry data, suggest different sources for the two rock types. The biotite granites are of purely crustal origin, derived by partial melting of a Paleoproterozoic amphibolitic lower continental crust, while a mixture of melts derived from the Paleoproterozoic continental crust and magmas derived from an enriched lithospheric mantle could explain the origin of porphyritic monzogranites. Based on the spatial and temporal distribution of magmatic rocks from the Alto Pajeú Domain, we suggest that a local lithospheric thinning model could explain the evolution of post-collisional magmatism in this region of the Borborema Province.

Platinum-Group Element Geochemistry of Igneous Rocks in the Chongjiang Cu–Mo–Au Deposit, Southern Tibet: Implications for the Formation of Post-Collisional Porphyry Cu Deposits

Abstract The timing and extent of sulfide saturation have been suggested as controlling factors in the formation of economically significant porphyry Cu deposits in subduction zone settings. However, details on the sulfide saturation history in post-collisional porphyry systems remain ambiguous. Accordingly, we have characterized the whole-rock geochemistry, including platinum-group elements (PGE), of igneous intrusions in the post-collisional Chongjiang porphyry Cu–Mo–Au deposit (southern Tibet) and utilize this data in conjunction with zircon U–Pb geochronological results and sulfide chemistry to assess the timing of sulfide saturation, the nature and amount of magmatic sulfide produced. The Chongjiang intrusions (monzogranite, biotite monzogranite porphyry, granodiorite, dacite porphyry, and quartz diorite porphyry) and mafic microgranular enclaves (MMEs) have zircon U–Pb ages of 14.2 to 12.8 Ma. Covariations in whole-rock major and trace elements among the Chongjiang intrusions and MMEs, together with similarities in their Sr–Nd and zircon Hf isotope compositions, indicate that they are co-magmatic and crystallized from a juvenile lower crustal melt that mixed with mafic melt derived from the lithospheric mantle; this hybrid melt subsequently evolved via fractional crystallization. Trace-element ratios in zircon and temperature − ∆FMQ estimates of the different intrusions suggest that they all crystallized from oxidized (average ∆FMQ = 1.9–2.6) and water-rich magmas. Palladium contents and Pd/Pt ratios in the Chongjiang igneous intrusions increase with decreasing MgO up to 3.9 wt % MgO, after which they abruptly decrease. The initial increase in Pd/Pt ratios likely results from the fractionation of a Pt-rich mineral (e.g. Pt–Fe alloy). The decrease in Pd contents and Pd/Pt ratios at 3.9 wt % MgO likely results from sulfide saturation during magma evolution, but prior to volatile exsolution, which occurred at approximately 1.4 to 2.4 wt % MgO. The presence of magmatic sulfide inclusions in amphibole and magnetite in samples with 3.9 wt % MgO, and the geochemical compositions of sulfide inclusions suggest that they represented trapped sulfide liquid and intermediate solid solution. Results of Monte Carlo simulations demonstrate that 0.003 to 0.009 wt % magmatic sulfide is required to have fractionated from the magma to explain the decrease in Pd contents at 3.9 wt % MgO. Highly chalcophile elements, such as Pd, will be sequestered by the magmatic sulfide that saturates at depth, decreasing their concentrations in the residual silicate melt, whereas concentrations of the less chalcophile elements, such as Cu, Mo, and even Au, will not be as significantly affected. Consequently, sufficient concentrations of Cu–Mo–Au will remain in the residual melt and, upon reaching volatile saturation, can be transported by the vapor phase to form porphyry Cu–Mo–Au deposits. In the case of the Chongjiang deposit, sulfide saturation was likely triggered by the high pressures and/or depletion of FeO caused by the thick (~70 km) crust beneath the Gangdese belt. This contribution presents evidence of sulfide saturation in post-collisional magmatic systems, and demonstrates that the amount of magmatic sulfide produced is a critical factor in controlling the formation of post-collisional porphyry Cu deposits.