Host Granodiorite Research Articles

Formation and degradation of a porphyry occurrence: The oligocene Khatoon-Abad porphyry Mo-Cu system, NW Iran

This study investigates the temporal relationships between mineralization and magmatism in the Khatoon-Abad porphyry Mo-Cu prospect (Urumieh-Dokhtar Magmatic Arc, NW Iran). Integrated zircon U-Pb and molybdenite Re-Os dating document a prolonged stationary magmatism, spanning ∼ 45 Myr (from ∼ 66 to ∼ 21 Ma; Paleocene-Early Miocene). Three main Oligocene ore-bearing granitic intrusions and an early Miocene barren dyke swarm are documented, with the main mineralization formed at ∼ 27 Ma, as attested by the molybdenite Re-Os age of 26.75 ± 0.14 Ma and the zircon U-Pb age 26.93 ± 0.30 Ma from the host granodiorite porphyry. Despite having similar geochemical fingerprints, including an adakitic signature and having REE patterns similar to productive magmas, the subsequent Oligocene granite bodies (∼26.0–25.7 Ma) yielded lower Mo-Cu enrichments and the early Miocene rhyodacite dykes (∼21 Ma) are barren. This evidence demonstrates that the efficiency of mineralization has been reduced by changes in the physiochemical conditions of magmatic-hydrothermal systems over time. We suggest that a perturbed geothermal gradient during later Oligocene granite (at ∼ 26 Ma) caused slow cooling/degassing of the melts, and hence determined an inefficient mineralization environment. We also infer that during the latest granite porphyry pulse (∼25.7 Ma), the structurally-controlled emplacement at shallower levels resulted in rapid melt cooling along with more meteoric water mixing, eventually minor potassic but vast phyllic alterations, and hence, causing a dispersed mineralization rather than a focused fluid flow. Therefore, the later Oligocene and early Miocene magmatic pulses degraded the early mineralization. The results of this study emphasize that a consistent magma supply into the chamber followed by a rapid magma-fluid flux to the mineralization site are needed for efficient mineralization in collisional settings. Otherwise multiple mineralization pathways and sites would result in low-grade ore bodies.

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.

Petrology and Geochemistry of an Unusual Granulite Facies Xenolith of the Late Oligocene Post-Obduction Koum Granodiorite (New Caledonia, Southwest Pacific): Geodynamic Inferences

Pressure–temperature estimates of a xenolith found within a post-obduction granodiorite in southern New Caledonia provide evidence for subcrustal, granulite facies, peak crystallisation conditions (ca. 850 °C—8.5 ± 1.0 kbar), followed by isobaric cooling to 700 °C, and final decompression with partial rehydration at ca. 650 °C—3.5 kbar. The xenolith, dated at 24.7 Ma (U-Pb zircon), i.e., the same age as the granodiorite host rock, has low SiO2 (35.5 wt%) and high Al2O3 (33.2 wt%) contents, suggesting that it is the restite of a previous melting episode, while the elevated Ca (Ba and Sr) contents suggest mantle metasomatism. Although the concentrations of Rb, K, Ca, Ba, and Sr have been strongly modified, some geochemical (REE patterns and some “immobile” trace element ratios) and isotopic (Sr and Nd isotopic ratios, U-Pb zircon age) characteristics of the granulite facies xenolith are similar to those of the xenoliths found in other Late Oligocene intrusions in southern New Caledonia; therefore, this rock is interpreted to be related to an early magmatic episode. The rock protolith was emplaced and equilibrated at the base of the crust where it underwent ductile deformation. Younger ascending magma picked it up and they eventually crystallised together at a shallow crustal level, near the tectonic sole of the ophiolite. The recrystallisation and ductile deformation at ~8.5 kbar suggest that a rheological discontinuity existed at about 25–28 km, probably representing the Moho. It is concluded that a continental crust of normal thickness must have existed beneath New Caledonia at about 24 Ma, i.e., 10 Ma after obduction.

The pristine precursor of Andean-type magmatism preserved in magma mingling zones

Intermediate magma compositions have been postulated to be parental to Andean-type magmatism in the recent years. Geochemical and experimental methods have allowed the modelling of a hypothetical parental composition that accounts for the major element trends displayed by Andean-type batholiths. However, natural plutonic examples matching the modelled composition remain lacking, likely due to the predominance of fractionated liquids and cumulates in the batholiths after protracted and large-scale differentiation. Contrary to this, magma mingling zones, a common feature in Andean-type batholiths, are characterised by quenching phenomena, minimising differentiation. In this paper, we present data from intermediate magmatism in the world-class Gerena magma mingling zone in the Seville Sierra Norte batholith (southern Iberia), compositionally equivalent to Andean-type magmatic series. Geochemical data from quenched dark globules of variable scale and the corresponding host granodiorites are contrasted with the bimodal trends displayed by the host batholith. Results suggest that the smaller-scale dark globules have not undergone any significant fractionation. Furthermore, after conducting geochemical modelling we conclude the dark globules represent a composition that could be parental to Andean-type magmas. We propose that magma mingling zones are an optimal place to probe for parental magmas of Andean-type magmatism, particularly those represented in pristine melanocratic, intermediate globules.

Petrogenesis of Middle Triassic Mafic Enclaves and Host Granodiorite in the Eastern Kunlun Orogenic Belt, NW China: Implications for Continental Crust Growth in Syn-Collisional Setting

Petrogenesis of Middle Triassic Mafic Enclaves and Host Granodiorite in the Eastern Kunlun Orogenic Belt, NW China: Implications for Continental Crust Growth in Syn-Collisional Setting

Superimposed Gold Mineralization Events in the Tuanshanbei Orogenic Gold Deposit, Central Jiangnan Orogen, South China

Abstract The Jiangnan orogen, one of the largest gold-producing areas in China, has experienced multiple orogenic events with complex structural overprinting that is marked by multiple stages of magmatism, deformation, metamorphism, and orogenic gold mineralization. Different orogenic events have been recognized in the Neoproterozoic, mid-Paleozoic, Triassic, and Early Cretaceous, reflecting collisions and intracontinental orogenic episodes. The age of gold deposition in the Jiangnan orogen, however, has been poorly constrained owing to the absence of suitable dating minerals. Field studies in the orogen indicate the Tuanshanbei gold deposit includes two generations of auriferous quartz-ankerite-pyrite-arsenopyrite veins (Q2 and Q3), with the latter of the two notable for containing more abundant ankerite and base metal sulfides. The Q2 veins were formed throughout the near S-N–directed shortening associated with D1 deformation and along resulting subhorizontal to low-angle-dipping EW- to WNW-striking transpressive faults. The Q3 veins, containing about 70% of the total gold resource, were primarily localized in moderately to steeply dipping NW-striking tensional/tensional shear faults and moderately dipping NE- to NNE-striking transpressive faults that were products of NW-SE–directed shortening during D2 deformation. Both vein generations are temporally younger than the 437.2 ± 4.2 Ma Tuanshanbei granodiorite host, and both are crosscut by postgold ca. 225 Ma diabase dikes. Hydrothermal monazite coexists with native gold and gold-bearing metal sulfides in the Q2 and Q3 veins. The Q2 monazite yielded a Tera-Wasserburg lower intercept age of 415.1 ± 2.1 Ma, consistent within error with an ankerite Sm-Nd isochron age of 410 ± 15 Ma and a laser ablation-inductively coupled plasma-mass spectrometry hydrothermal zircon 206Pb/238U age of 411.2 ± 4.0 Ma. The Q3 monazite yielded a Tera-Wasserburg lower intercept age of 234.3 ± 1.1 Ma. These new ages suggest that the Early Devonian gold event was overprinted by hydrothermal activity along the same structural system almost 200 m.y. later such that the gold resource must be a product of two temporally distinct events. Geologic and structural evidence, coupled with existing published geochemical data, suggests both ore-forming events were related to crustal metamorphism typical of most orogenic gold deposits. Fluids would have been derived from Neoproterozoic metasedimentary basement rocks, most likely from metamorphic devolatilization of the Neoproterozoic Cangxiyan Group greenschist-amphibolite facies metasediments. There is no evidence suggesting any type of magmatic contribution to the ore-forming process at either time. The data are best interpreted to suggest that various parts of the basement were metamorphosed near the greenschist-amphibolite boundary at different times, but during both times, the gold-bearing metamorphic fluids eventually migrated into the same structural conduits.

Tearing on the southward subducting Kelameili oceanic lithosphere in the early Devonian: Evidence from the magmatism in the Harlik arc, southern Altaids

The coexistence of adakites, high-Mg andesites (HMAs) and Nb-enriched basalts (NEBs) records special subduction geodynamics, for instance, subduction initiation, subduction of oceanic ridges or slab tearing, which are widespread in the circum-Pacific subduction zone. The Altaids in Central Asia shared similar tectonic evolution to the western Pacific. However, slab tearing has not been widely reported, which hinders a better comparison between the Altaids and the western Pacific. In this study, we report the contemporaneous early Devonian granodiorites and mafic microgranular enclaves (MMEs) in the Sidingheishan area of the Harlik arc, southern Altaids. The granodiorites have high Sr (534–683 ppm) but low Y (7.79–10.2 ppm) and Yb (0.71–1.68 ppm) contents, with corresponding high Sr/Y values (29.78–76.93). In combination with their juvenile isotopes (initial 87Sr/86Sr = 0.70333–0.70356, εNd(t) = +7.4 − +8.01) and high Mg# numbers (50.9–53), they indicate that their primitive magmas were derived from slab melting. The MMEs show similarly high Sr contents (507–696 ppm), and identically juvenile isotopic signatures (initial 87Sr/86Sr = 0.70346–0.70356, εNd(t) = +7.14 − +7.5) to their host granodiorites. However, their relatively low silica contents (55–60 wt%), evolved Mg# numbers (44.8–55.2) and high Nb contents (12.7–24.7 ppm) demonstrate that they are the fractionation products of NEBs, which were produced by the partial melting of the mantle that was previously metasomatized by these adakitic magmas. Considering all the available evidence, we propose a slab tearing model in which the southward subducting Kelameili oceanic lithosphere underneath the Harlik arc underwent rollback in the early Devonian and its differential rollback rates caused slab tearing. Subsequent asthenosphere upwelling stimulated the Kelameili oceanic lithosphere to melt, producing the adakitic magmas, some of which intruded the crust after limited interaction with the mantle peridotites and underwent crystallization of dominant amphibole in the crust. The rest reacted completely with the mantle peridotites, which underwent further melting to produce the Nb-enriched basaltic magmas. They intruded into the adakitic magma chamber after fractional crystallization of olivine, clinopyroxene and garnet in the deep, forming the MMEs and promoting the remobilization of those hornblendites. This study reveals the development of slab tearing that are closely associated with slab rollback in the Altaids, providing further evidence that argues for the similarity between the Altaids and the western Pacific.

Coupled alkaline high-Nb mafic rocks and adakitic granodiorites: Products of Neoproterozoic back-arc extension at the western margin of the Yangtze Block, South China

The newly identified Neoproterozoic Yuanmou high-Nb mafic and adakitic rocks at the western margin of the Yangtze Block in South China are investigated for petrogenesis and geodynamic implications. Zircon U-Pb ages show the adakitic granodiorite host and mafic microgranular enclaves (MMEs) were synchronously emplaced at 811−806 Ma, and were cut by an 802 ± 11.5 Ma mafic dike. The medium-grained granodiorites consist mainly of plagioclase, quartz, K-felspar, biotite, and amphibole. High SiO2 (62.8−67.5 wt%), Al2O3 (13.1−16.8 wt%) contents, high Sr/Y (17.4−49.0), and (La/Yb)N (16.3−52.6) but low Y (6.83−16.1 ppm) and Yb (0.74−1.82 ppm) concentrations of the granodiorites point toward their calc-alkaline and adakitic nature. Further, their high Dy/Yb (1.52−2.31) and Gd/Y (2.35−4.43) and low K2O/Na2O (0.22−0.48) and Th/La (0.14−0.20) ratios, constant bulk rock εNd(t) (−0.5 to −1.5) and zircon εHf(t) (0.0 to +2.3) values, suggest that the granodiorites were derived from partial melting of a mafic lower crustal source. The MMEs in the granodiorites and mafic dike intrusions are composed of amphibole, plagioclase, clinopyroxene, and olivine, and have similar chemical compositions, such as high Nb (15.7−41.9 ppm) and TiO2 (2.13−3.39 wt%) contents and high Nb/U (18.7−30.9) values, akin to alkaline high-Nb basalts (HNB). Their variable Ba/Nb (7.68−79.2), Nb/Zr (0.09−0.16), high Nb/Th (5.47−6.68), and low Th/Zr (0.01−0.03) and La/Nb (1.03−1.37) ratios indicate a mantle source modified by melts derived from subducted oceanic crust and sediments. The Yuanmou HNB with high εNd(t) (+4.8 to +6.9) were probably generated during back-arc extension by decompression melting of a mantle source metasomatized by adakitic slab melts formed during earlier stages of the subduction. This implies that the location of the back-arc was far from the subducted oceanic slab, possibly due to a slab rollback process. In this scenario, HNB intrusions provided the heat for partial melting and adakitic melt formation in the mafic lower crust. The model suggests that the Yuanmou and other similar associations of HNBs and adakitic rocks emplaced along the western margin of the Yangtze Block are products of a widespread Neoproterozoic back-arc extension.

Late Neoarchean crust–mantle interaction and tectonic implications in western Shandong Province, North China Craton: Evidence from a granodiorite pluton and associated magmatic enclaves

This study presents original data on the microstructure, mineralogy, whole–rock geochemistry, and isotopes of a Late Neoarchean granodiorite pluton and associated mafic microgranular enclaves (MMEs) in western Shandong Province of the eastern part of the North China Craton (NCC). The MMEs hosted in the granitic pluton are more mafic, and amphibole and biotite are the predominant mafic minerals. The sharp contacts and fine–grained textures of the MMEs indicated a rapid quenching process during a magma mingling event. Zircon and titanite UPb dating indicated that the host granodiorite pluton formed at circa 2546 Ma, and the MMEs formed at approximately 2538 Ma. The dating results indicated a crust–mantle interaction. Geochemically, the host pluton has high silica contents (SiO2 = 67.68–68.51 wt%), Mg# values (51–53) and low Aluminum Saturation Index ratios (A/CNK = 0.92–0.95), which are indicative of I–type granitic affinity. Moreover, they show arc–type trace element features characterized by enrichment of light rare earth elements (LREEs) and large ion lithophile elements (LILEs) (e.g., Cs, Rb, K, and Pb), but depletion in high field strength elements (HFSEs) (e.g., Nb, Ta, P, and Ti). In contrast, the MMEs have lower silica contents (SiO2 = 52.07–56.03 wt%), but higher Mg# values (56–68) and Cr, Ni and Cu contents of 454–633, 159–225 and 98.3–207 ppm, respectively, corresponding to a mantle component. Similar to the host pluton, the MMEs show enrichment of LREEs and LILEs, but depletion of HFSEs, reflecting an arc–type magma origin. Isotopically, the host granodiorite and MMEs both have relatively depleted NdHf isotopic compositions, with εNd(t) values of −0.67–1.22 and − 0.31–1.89, and εHf(t) values of 5.63–2.03 and − 0.18–4.48, respectively. Zircon oxygen isotopes reveal that the host granodiorite and MMEs have slightly higher oxygen isotopes than the normal mantle zircon (5.3‰), with mean δ18O values of 6.35‰ and 6.81‰, respectively, indicating assimilation of minor supracrustal materials in the magma source. However, a combination of Nd–Hf–O isotopes suggests that the MMEs haveslightly enriched isotopic compositions than the host granodiorite pluton, suggesting a “reversed isotope” feature. This important finding also enlightens that “reversed isotope” phenomenon represented magma mingling between isotopically relatively enriched mafic and relatively depleted felsic magmas. Furthermore, the Ce4+/Ce3+ ratios in zircon from the host granodiorite and MMEs indicated a low oxygen fugacity, with mean values of 24.25 and 39.69, respectively. Although the host granodiorite and MMEs have the similar oxygen fugacity ranges, the MMEs have high Cu concentrations of 98.3–207 ppm (mean value = 151 ppm), indicating a Cu–rich magma source. Integrating the petrogenesis of the Late Neoarchean granodiorite pluton and associated MMEs, it is inferred that the host granodiorite pluton was derived from partial melting of mafic lower crust with minor insignificant ancient materials, and the MMEs were derived from partial melting of depleted mantle metasomatized by sulfur–rich sediment melts/fluids (chalcophile element). Tectonically, our samples reveal an affinity with an arc in a syn–collisional setting. Together with regional rock assemblages, we suggest that the onset of plate tectonics (subduction) was to occur by the end of Neoarchean in the NCC.

Petrogenesis and Tectonic Implications of the Triassic Granitoids in the Ela Mountain Area of the East Kunlun Orogenic Belt

The East Kunlun Orogenic Belt is located in the western part of the Central Orogenic Belt of China, with a large number of Triassic igneous rocks parallel to the Paleo-Tethys ophiolite belt, which provides a large amount of geological information for the tectonic evolution of the Paleo-Tethys Ocean. The granitoids studied in this paper are located in the Ela Mountain area in the eastern part of the East Kunlun Orogenic Belt. Zircon U-Pb dating results show that these different types of granitoids were crystallized in the Triassic. The 247.5 Ma porphyritic granites from Zairiri (ZRR) displayed calc-alkaline I-type granite affinities, with the zircon εHf(t) values being mainly positive (−0.5 to + 3.8, TDM2 of 1309–1031 Ma), indicating that they are derived from the partial melting of the juvenile crust and mixed with ancient crustal components. The 236.8 Ma Henqionggou (HQG) granodiorites and 237.5 Ma Daheba (DHB) granodiorites are high-K calc-alkaline I-type granite, and both have mafic microgranular enclaves (MMEs), showing higher and more varied Mg# (39.73–62.73), combined with their negative Hf isotopes (εHf(t) = −2.6 to −1.6, TDM2 = 1430–1369 Ma), suggesting that their primary magmas were the products of partial melting of the Mesoproterozoic lower crust that mixed with mantle-derived rocks. The 236.4 Ma DHB porphyritic diorites showed characteristics of high-K calc-alkaline I-type granitoids, with moderate SiO2 contents, medium Mg# values (40.41–40.65), with the Hf isotopes (εHf(t) = −2.9 to −0.5; TDM2 = 1451–1298 Ma) indistinguishably relative to contemporaneous host granodiorites and MMEs. The petrographic and geochemical characteristics indicate that the porphyritic diorites are the product of well-mixed magma derived from the Mesoproterozoic lower crust and lithospheric mantle. Based on the results of this paper and previous data, the chronology framework of Late Permian–Triassic magmatic rocks in the eastern part of the East Kunlun Orogenic Belt was constructed, and the magmatic activities in this area were divided into three peak periods, with each peak representing an extensional event in a particular tectonic setting, for example, P1 (slab roll-back in subduction period; 254–246 Ma), P2 (slab break-off in transition period of subduction and collision; 244–232 Ma), P3 (delamination after collision; 230–218 Ma).

Petrogenesis of Early Cretaceous granitoids and mafic enclaves from the Jiaodong Peninsula, eastern China: Implications for crust-mantle interaction, tectonic evolution and gold mineralization

Early Cretaceous granitic plutons in the Jiaobei Terrane, Eastern China, contain abundant mafic enclaves (MMEs), which are important to understand the tectonic settings and genesis of gold mineralization in the Jiaodong Peninsula. Here, we combine petrology, geochronology, and isotopic geochemistry for MMEs and host granitoids to constrain their tectonic evolution and understand the genesis of coeval gold mineralization in the Jiaodong Peninsula during the Early Cretaceous. Zircon U-Pb dating reveals two magmatic events: (1) the earlier magmatic event (130–126 Ma), represented by Congjia MME (CJ-MMEs) and host granodiorite, and (2) the later magmatic event (121–116 Ma), including Gushan MME (GS-MMEs) and host granite. Field relationship, disequilibrium textures, and elemental and Sr-Nd-Hf isotopic compositions indicate that MMEs and host granitoids formed through mixing/mingling of mafic and felsic magmas. However, significant differences between CJ-MMEs and GS-MMEs indicate that the former has adakitic affinity and was produced by mixing of magmas from lithospheric mantle and mafic lower crust in deep hot zone, whereas the latter has arc-magmatic affinity, suggesting enriched lithospheric mantle source. Higher SiO2 and lower MgO contents in the host granitoids indicate that they were originated from reworking ancient NCC lower crust. Our results reveal crustal growth during the earlier magmatic event, followed by rapid crustal/lithospheric thinning in the later magmatic event, all triggered by ongoing rollback of subducting Paleo-pacific plate. These results combined with complex assemblages of sulfides and volatiles in GS-MMEs indicate that the lithospheric thinning, the rise of enriched lithospheric mantle-derived melts to shallow crustal levels, and crustal-mantle interactions around ∼ 120 ± 3 Ma, are the main factors for massive gold precipitation in the Jiaodong Peninsula.

Petrogenesis and geodynamic significance of Late Triassic mafic microgranular enclaves and its host granodiorite in the Shuanghu area, central Qiangtang

The genesis of Late Triassic granitoids in the central Qiangtang is significant for deducing the evolution of the Palaeo‐Tethys Ocean in Qiangtang Block. Herein, we present a comprehensive study of zircon U–Pb ages, whole‐rock geochemistry and zircon Lu–Hf isotopes for the mafic microgranular enclaves (MMEs) and host granodiorites from the Shuanghu area, in order to evaluate their petrogenesis and geodynamic implications. New zircon U–Pb dating show identical formation ages for host granodiorites (207.1 ± 2.2 Ma and 206.6 ± 3.8 Ma) and MMEs (205.4 ± 2.9 Ma and 209.2 ± 2.2 Ma). The Shuanghu granodiorites contain amphibole and biotite and are characterized by moderate SiO2 (64.74–66.40 wt%) and K2O contents (2.48–3.62 wt%) as well as low A/CNK (0.69–1.00), indicating I‐type granite affinity. Its enriched zircon Hf isotopes (εHf(t) = −12.04 to −5.09) indicate that they were derived from the ancient mafic lower crust. The MMEs have a similar mineral assemblage, emplacement ages, trace elemental and zircon Hf isotopic compositions with the host granodiorites, indicating the cognate origin. These MMEs represent autoliths captured by latter ascending host magmas in the middle‐upper magma chamber. The highly variable Mg# (37–53), Cr (16.9–190 ppm) and Ni (5.71–67.2 ppm) in the Shuanghu MMEs and host granodiorites indicate that the mantle‐derived magma provided not only the heat but also the mafic components. In combining with previous studies, we suggest that the Shuanghu granodiorites and MMEs were formed in the post‐collision stage, which were caused by the asthenosphere upwelling related to the slab breakoff.

The timescale of solid-state deformation in the Northern Adamello igneous intrusive suite

The Cenozoic Adamello batholith in the Southern Alps records solid-state deformation structures including, in order of decreasing relative age, cooling joints, shear zones and faults. In the present study we constrained age and duration of each phase with 40 Ar/ 39 Ar geochronology. The host granodiorite, cooling joints, mylonites, pseudotachylytes and cataclasites were characterized through microstructural and mineralogical analysis, micro-computerized tomography and electron probe microanalysis. The dated K-bearing phases are (1) magmatic biotite, (2) biotite and K-feldspar in joints and mylonites, (3) pseudotachylytes and (4) hydrothermal K-feldspar in cataclasites. The wall-rock biotite has an age of 33.2 ± 0.2 Ma, independent of grain size, overlapping with the age of the cooling joints. Bulk biotite-rich mylonites have ages between 32.4 ± 0.5 and 30.8 ± 0.08 Ma. The K-feldspar cementing cataclasite has an age of 25.5 ± 1.1 Ma. Pseudotachylyte ages cluster between 29.8 ± 0.3 and 31.7 ± 3.1 Ma, with one of 25.6 ± 0.3 Ma. The resolvable difference in age between magmatic biotite and mylonites indicates that biotite is not a thermochronometer, as its age is mostly controlled by deformation and fluid–rock interactions. 40 Ar/ 39 Ar dates mostly confirm the relative ages determined from field relationships, with mylonites active within a time window of 1.6 myr and subsequent seismic faulting occurring for almost 6 myr. Supplementary material: Full analytical data are available at https://doi.org/10.6084/m9.figshare.c.5838146 Thematic collection: This article is part of the Isotopic Dating of Deformation collection available at: https://www.lyellcollection.org/cc/isotopic-dating-of-deformation

Variscan granitic magmatism in the Western Carpathians with linkage to slab break-off

The Variscan basement within the Western Carpathian Alpine architecture generally consists of metaluminous/peraluminous tonalite/granodiorite massifs and high-grade metamorphic complexes of metapelites, metaultramafites, and metabasites with relics of eclogites. Unfortunately, the Variscan crystalline basement of the Western Carpathians is only fragmentally exposed. Therefore, the proposed geodynamic evolutionary model for the Variscan granites of the Western Carpathians is primarily based on granite data from the Malá Fatra Mts. with additional dating from the High Tatra Mts. The oldest magmatic age of 362 ± 4 Ma in the Malá Fatra horst was recorded in diatexites from a high-grade metamorphic complex, which is related to crustal anatexis during Variscan subduction. Subsequent collisional event and break-off of the subducted slab promoted exhumation of the diatexites within the high grade metamorphic complex and intrusion of 353 ± 3 Ma old Tournaisian tonalite. Intensive heat input after slab break-off from the rising asthenosphere generated melting of the lower crust and extensive calc-alkaline, Mg-rich granitic magmatism in a short time span from 347 ± 4 to 342 ± 3 Ma. These Visean granitic rocks caused thermal overprint on the roof metamorphic rocks, including diatexites and Tournaisian tonalites at ca. 348 ± 5.6 to 342 ± 3 Ma. The Visean granite formation was controlled by the mixing of hot magmas, which is indicated by the presence of composite oligoclase/andesine plagioclase with preserved labradorite cores, alkali feldspars with Na2O ≥ 2 wt%, zoned apatite, the presence of antiperthite, and quartz ocelli. Elevated contents of mantle-derived elements like V, Ni, Cr, Ba, high Sr/Y ratio of ~44, steep LREE and flat HREE segments of chondrite-normalised patterns document adakite-like feature of the investigated granitic rocks which resulted from melting of a mixed lower-crustal and mantle sources and crystallisation in the presence of garnet. Unusual abundance of Fe–Ti oxides in granodiorites with magmatic cooling temperatures of 735–756 °C supports high-T input from mantle. In the High Tatra Mts., diorite xenolith shows the age of 359.2 ± 3 Ma, and its host granodiorite the age of 350.1 ± 2.6 Ma. The diorite contains acicular zircons, which points to rapid exhumation. Stubby zircon of the host granodiorite shows regular, oscillatory zoning controlled by a gradual temperature decrease. The non-comagmatic relationships between diorite and host granodiorite are indicated also by a difference in zircon Th/U ratio, which is 0.2 for the host granodiorite, but 1.0 for the diorite on average. The presented data show that slab break-off could have been a mechanism that promoted Variscan granitic magmatism in the Western Carpathians.

Late Indosinian granitoids in the Zhacanggou, Guide Basin, NW China: Whole‐rock geochemistry, zircon U–Pb geochronology, Lu–Hf isotopes, and implications for magma mixing

The Guide Basin, located in the West Qinling orogen of central China, is a typical geothermal field. However, the granodiorites occurring in the Zhacanggou area have been poorly restricted. Herein, we report new isotopic compositions of whole‐rock major‐trace elements, zircon U–Pb, and Lu–Hf, on the host granodiorites and their microgranular enclaves (MEs) in the Zhacanggou area. Zircon U–Pb dating for the host rocks (porphyritic granodiorite and granodiorite) revealed ages of 230.5 ± 1.4 Ma and 229.0 ± 1.6 Ma, similar to those (230.5 ± 1.5 Ma and 231.4 ± 1.4 Ma) of the MEs. Zircon Hf isotope analyses for the granodiorite and MEs yielded εHf(t) values from −11.0 to −1.5 and from −7.8 to −0.7, with corresponding TDM2 ages of 1,355–1,959 and 1,304–1,755, respectively. The MEs possess plagioclase phenocrysts, acicular apatites, and chilled borders. They were characterized by relatively low SiO2 (58.39–65.24 wt%) and high Cr, Ni, and MgO contents, leading to relatively high Mg# values (44–59) and low Sr/Y (14.23–19.84) ratios. The host granodioritic rocks show similar adakitic geochemical characteristics with SiO2 = 64.23–68.57 wt%, MgO = 1.29–2.22 wt%, Mg# = 48–53, Sr = 340–596 ppm, Y = 11.3–13.7 ppm, Sr/Y = 28.39–47.89, and no obvious Eu anomalies. These geochemical and isotopic data, combined with the mineralogical features, indicate that the granodiorites in the Zhacanggou area were produced by the mixing of high‐Mg# melts originating from an enriched lithospheric mantle source, and felsic magmas deriving by the partial melting of the normal lower continental crust with the tectonic regime transition from a syn‐collision to a post‐collision setting.

Identifying Crystal Accumulation in Granitoids through Amphibole Composition and In Situ Zircon O Isotopes in North Qilian Orogen

Abstract Granitoids are the main constituents of the continental crust, and an understanding of their petrogenesis is key to the origin and evolution of continents. Whether crystal fractionation is the dominant way to generate evolved magmas has long been debated, mostly because such processes would produce large volumes of complementary cumulates, which remains elusive. Mafic magmatic enclaves (MMEs) are ubiquitous in granitoids and their presence was initially recognized as cumulates. However, because many MMEs lack obvious evidence of accumulation, such as the classic cumulate textures and modal layering, the cumulate origin of MMEs has been abandoned and the model of magma mixing between mafic and felsic magmas has become popular. In this study, we conduct a combined study of amphibole composition and in situ O isotopes in zircons on three suites of orogenic granitoids with MMEs from the North Qilian Orogenic Belt (NQOB). We find that the MMEs and their host granodiorites show overlapping zircon δ18O values, affirming that they share the same parental magmas. The amphibole compositions indicate that amphiboles from the MMEs are not in equilibrium with a melt whose composition was that of the bulk-rock. These new data, together with the published bulk-rock data, suggest that the MMEs in our study have clear cumulate signatures and are thus of cumulate origin. Our study provides evidence for crystal accumulation in granitoids in the NQOB. This new understanding calls for re-examination on the petrogenesis of some intermediate magmatic rocks (granitoid/andesite) in discussing models of continental crustal growth.

Magmatic controls on the mineralization potential of a porphyry Cu system: The case of Jurassic Tongshan skarn Cu deposit in the Qin–Hang Belt, South China

The Qin–Hang suture belt in South China formed during the Neoproterozoic amalgamation between the Cathaysia and Yangtze blocks. There are several Jurassic porphyry–skarn Cu deposits in this region, and the factors controlling the mineralization potential remain unclear. This study reports geochemical data for the Tongshan skarn deposit and zircon trace element data for five porphyry- or skarn-type Cu deposits in this region, in order to identify geochemical indexes for the mineralization potential of the ore-related intrusions. A garnet U–Pb age (170 Ma) for the Tongshan skarn indicates that Cu mineralization occurred in the Middle Jurassic, which is consistent with the zircon U–Pb ages (173 Ma–171 Ma) of the host granodiorite porphyries. Granodiorite porphyry samples from the Tongshan deposit have moderately high SiO2 contents (64.4–65.0 wt%), low MgO contents (1.39–1.47 wt%), and high (La/Yb)N ratios (14.7–16.8), typical of low-Mg adakitic rocks. The whole-rock geochemistry, bulk Earth-like εNd (t) values (–1.5 to –1.6) and positive zircon εHf(t) values (+2.7 to +6.0), corresponding to two-stage Hf model ages of 1014–832 Ma, suggest the porphyries were derived from thickened juvenile lower crust initially generated during Neoproterozoic amalgamation. In addition, the porphyry samples have arc-like geochemical characteristics (Nb–Ta–Ti depletion), low Th/Yb ratios (average = 4.7), high Ba/Th ratios (average = 346), zircon Eu/Eu* values (average = 0.71) and 10000×(Eu/Eu*)/Y values (average = 6.80), and moderate zircon Ce/Ce* values (average = 705) and Ce/Nd ratios (average = 17.1). These features demonstrate that the parental magmas of the porphyries were formed by melting of a mantle wedge.That had been metasomatized by moderately oxidized slab-derived fluid near a Neoproterozoic subduction zone. Previous studies suggest porphyries from large to giant deposit (e.g., Dexing and Yinshan) have similar two-stage Hf model ages and were derived from Neoproterozoic juvenile crust. But these porphyries have higher whole–rock Th/Yb ratios and zircon Ce/Ce* and Ce/Nd values than those of intrusions from medium-sized deposits (e.g., Tongshan, Chuankeng and Jiande). This indicates that the magma source of the former may have been distant from the trench in the Neoproterozoic, and formed by melting of a mantle wedge that had been metasomatized by highly oxidized slab-derived melt. Our results and previously published data indicate that the Neoproterozoic two-stage Hf model ages of the intrusions are positive indicators of Jurassic Cu mineralization, and granitoids with relatively high Th/Yb ratios and zircon Ce/Ce* and Ce/Nd values likely have more potential to form large to giant Cu deposits.

The Potential for Low‐Grade Metamorphism to Facilitate Fault Instability in a Geothermal Reservoir

AbstractNative and fluid circulation‐induced metamorphic products may affect the stability of faults in geothermal reservoirs–particularly epidote (Ep) and chlorite (Cl). Our laboratory experiments conducted at hydrothermal conditions show that these two minerals, when precipitated on natural faults, may promote fault instability or failure under conditions typifying a geothermal reservoir. Shear experiments on Ep‐rich fault gouges indicate potentially unstable frictional behavior—more pronounced at elevated temperatures and pore fluid pressures—and indicate increased potential for the nucleation of slip instability relative to that of the host granodiorite gouge. Experimental results indicate in particular that Ep can increase the potential for induced seismicity on existing faults. Increased proportions of Cl in gouges stabilize the faults but apparently lower the frictional strength. Our results reveal that the metamorphic minerals exert a subtle but potentially significant control on fault strength and stability in geothermal reservoirs.

Genesis of Mafic Microgranular Enclaves in the Shaocunwu Granodiorite, Southern China, and their Implications: Evidence from Zircon and Whole‐rock Geochemistry

AbstractMagmatic microgranular enclaves (MMEs) are widely developed in the Shaocunwu granodiorite at the northeast margin of the eastern Jiangnan orogenic belt. Field geology showed that the MMEs occur as irregular ellipsoids near the edge of the intrusion, and consist of diorite, dominantly composed of amphibole, biotite, and plagioclase grains, with minor acicular apatite. Zircon U‐Pb dating showed the ages of the host granodiorites and MMEs are 145.9 ± 1.1 Ma and 145.6 ± 2.5 Ma, respectively, indicating both originated during coeval late Jurassic magmatism. Whole‐rock geochemical results show that the host granodiorite and MMEs have similar rare earth and trace element partition curves in spider grams, and similar 87Sr/86Sr, and 147Nd /144Nd isotope ratios, and their zircon 177Hf/176Hf isotopic ratios are similar. Geochemical studies indicate that both the host granodiorite and MMEs formed by mixing of coeval magma. Zircon Ti thermometers and oxygen fugacity of the host granodiorite and the MMEs show high oxygen fugacity, similar to that of W‐Cu (Mo) mineralized granitoids in the eastern Jiangnan orogenic belt. A similar magma mixing process was probably one of the mechanisms that generated the W‐Cu (Mo) fertile melts.

Linking ocean subduction with early Paleozoic intracontinental orogeny in South China: Insights from the Xiaying complex in eastern Guangxi Province

The tectonic setting of the early Paleozoic orogeny in the South China Block (SCB) has been hotly controversial. Although most studies have ascribed it to an intracontinental orogeny, there are various views on the geodynamic drivers of the intracontinental orogeny. This study reports on the Ordovician Xiaying igneous complex in the eastern part of Guangxi province, southern South China. This complex is composed of granodiorite, monzogranite and micro-granular diorite enclosed by granodiorite. The zircon U–Pb dating suggests that these three types of rocks emplaced synchronously at around 464–442 Ma, significantly older than those S- and I-type granites (450–400 Ma) distributed in the interior of the SCB. One monzogranite collected in the border of the complex has an older age (480 Ma). The Xiaying hornblende–bearing granodiorites have low SiO2 (61.8–68.7 wt%), P2O5 (0.07–0.10 wt%), Rb/Sr (0.52–1.30) and Rb/Ba (0.13–0.34) and high Fe2O3t (3.84–5.57 wt%), MgO (1.48–2.35 wt%), CaO (2.28–4.54 wt%) and Na2O/K2O (0.90–1.91), with metaluminous to weakly peraluminous features (most A/CNK = 0.87–1.07), showing typical I-type granitoid characteristics. The Nd–Hf isotopic model ages (ca 1.6–1.8 Ga) indicate that the Xiaying granodiorites were probably formed by partial melting of a late Paleoproterozoic basement in the deep crust. The micro–granular dioritic enclaves (MMEs) have similar formation ages and Nd–Hf isotopic compositions to the host granodiorites, and show geochemical evolution relation with granodiorites, suggesting that they resulted from the mixing of mafic and granodioritic magmas, which derived from a depleted mantle and a Paleoproterozoic crust, respectively. Based on their similar formation ages and zircon Hf-isotope compositions and the assimilation and fractional crystallization (AFC) modelling, it is suggested that major monzogranites were generated from the granodioritic magma through the AFC process. The rock associations and arc-related geochemical characteristics of the granodiorites and dioritic MMEs suggest that they probably formed in a subduction-related tectonic setting. Combined with the spatial distribution of the rock types and formation ages of these early Ordovician to early Devonian magmatic rocks in the SCB from southeast to northwest, it is inferred that there was an Ordovician northwestward subduction of the proto–South China Ocean occurring to the south of the SCB, which may connect with the Tam Ky–Phuoc Son Suture Zone in the eastern Indochina Block. It was the subduction of the proto–South China Ocean and subsequent collision between the southern SCB and an unknown continent that triggered early Ordovician to early Devonian intracontinental orogeny in the interior of the SCB.