Typical Adakites Research Articles

Petrogenesis of Early Cretaceous volcanic rocks from the Rena-Co area in the southern Qiangtang Terrane, central Tibet: evidence from zircon U-Pb geochronology, petrochemistry and Sr-Nd-Pb–Hf isotope characteristics

AbstractThe subduction of the Bangong–Nujiang Ocean is important in the geological evolution of the Tibetan Plateau. In this paper, we report new zircon U-Pb age and Lu-Hf isotopic data and whole-rock elemental and Sr-Nd-Pb isotopic data for Early Cretaceous dacites from the Rena-Co area (RCA) in the southern Qiangtang Terrane (QT), central Tibet and use these data to better understand the tectonic evolution of the Bangong–Nujiang suture. LA–ICP-MS dating of zircons yields ages of 109.5 ± 0.6 Ma to 109.6 ± 0.8 Ma for the dacites from the RCA. Geochemically, these dacites are medium-K calc-alkaline and show high SiO2 contents of 64.79–70.37 wt.%, high Sr contents of 517–598 ppm and low Y contents of 8.45–10.7 ppm, similar to those of typical adakites. Additionally, all the rocks are strongly enriched in light rare earth elements and some large ion lithophile elements (e.g. Rb, U, K and Cs) but significantly depleted in high-field-strength elements (e.g. Nb, Ta and Ti), consistent with the geochemical characteristics of arc-type magmas formed in the subduction zone. Moreover, these adakite-like dacites show whole-rock initial (87Sr/86Sr)i ratios of 0.705119 to 0.705491, (206Pb/204Pb)i ratios of 18.489 to 18.508, (207Pb/204Pb)i ratios of 15.591 to 15.612, (208Pb/204Pb)i ratios of 38.599 to 38.686, ϵNd(t) values of −0.28 to +1.25 and single-stage Nd model ages of 642 to 818 Ma, as well as significantly positive zircon ϵHf(t) values of 3.9–13.1, with young Hf-depleted mantle ages of 331 to 923 Ma. These geochemical and isotopic data indicate that they are most likely derived from the juvenile thickened mafic lower continental crust, which contains partial melts of metasomatized peridotite and subduction-related fluids in the magma source region. Based on previous studies and our new data, we propose that the RCA adakite-like dacites are most likely a result of the northwards subduction of the Bangong–Nujiang Ocean lithosphere beneath the southern QT during the Early Cretaceous and that a slab rollback model could explain the formation of the RCA adakite-like dacites.

Geochronology, geochemistry and Hf isotopic compositions of late Silurian granodiorites in the Shaolang River metallogenic belt: Implications for magmatism and tectonic evolution

The Shaolang River metallogenic belt in Inner Mongolia is one of the main lead‐zinc polymetallic belt on the northern margin of the North China Plate. The granodiorites, which are closely coexisting with lead‐zinc veins in deposit, are extensively developed. Previous geochronological research results show that the dikes and veins in the metallogenic belt belong to the products of magmatic hydrothermal activity in the middle Yanshanian, so the genesis of the metallogenic belt deposit is generally considered to be a hydrothermal vein type deposit related to sub‐volcanic dikes. We obtained zircon LA‐ICP‐MS U–Pb ages and Hf isotopic compositions of granodiorites, which are widely distributed in the deposit and were considered to be closely connected with the genesis of the deposits. The zircon U–Pb ages are 419.4 ± 1.4 Ma and 424.5 ± 4.1 Ma for the granodiorites, respectively, and they formed in the late Silurian. The geological and geochemical data indicate that the granodiorites are consistent with the geochemical characteristics of typical adakite. All εHf(t) values of the granodiorites are minus, on the zircon Hf isotope age diagram, εHf(t) are below the evolution curve of chondrite. The two‐stage model ages (tDM2) are 2.45 ~ 2.79 Ga and 1.87 ~ 2.29 Ga, respectively, showing rock‐forming materials mainly derived from the Paleoproterozoic and earlier crustal partial melting, and there may have been mantle‐derived magma underplating of the lower crust. Combining with the previous regional research, it is concluded that the granodiorites were formed during the extinction of the island arc under the background of subduction of the Paleo‐Asian Ocean and North China Craton. Because of the 125 ~ 170 Ma metallogenic age, we concluded that the granodiorites in the ore areas have no obvious relationship with the deposit genesis. However, it provided the advantageous space for the formation of the lead‐zinc veins, it can be regarded as an important prospecting indicator in metallogenic belt.

Subduction of the Neo-Tethys ridge beneath the Eurasian continent during the Cretaceous

Ridge subduction usually has significant influences on the tectonics, magmatic activity and mineralization of the overriding plate because of its special thermal and topographic structures. The Neo-Tethys is a vanished ancient ocean, and thus, its spreading ridge must have been subducted beneath the Eurasian continent. However, where and when the subduction of the Neo-Tethys ridge occurred has not been well constrained. In this study, a Late Cretaceous (∼100 Ma) EW-trending igneous rock belt along the southern margin of the Eurasian continent from the Gangdese terrane to the South China block is recognized based on residual geological records and plate reconstruction. Importantly, these igneous rocks show the characteristics of typical adakites and are consistent with those of adakites derived from partial melting of subducted oceanic crust. Combined with the tectonic evolution of the Neo-Tethys Ocean, ridge subduction was most likely responsible for this adakitic igneous rock belt. In addition, a NS-trending compression event simultaneously occurred in the southern margin of the Gangdese terrane and the South China block during the Late Cretaceous, which agrees well with ridge subduction. The identification of the Neo-Tethys ridge subduction is of great significance for understanding the evolution, magmatism and mineralization of the Neo-Tethys tectonic domain.

Latest Oligocene adakitic rocks in western Iran: implications for early crustal thickening and tectonic evolution of the Iran Block

Adakitic rocks occur in a variety of tectonic settings and are key to understanding the tectonic evolution and geodynamics of orogenic belts. We investigated latest Oligocene (23.5–22.5 Ma) quartz monzonites and granites from the western segment of the Urumieh–Dokhtar magmatic belt in Iran, which are likely to have formed in response to the early stages of Arabia–Eurasia collision. The studied rocks have the geochemical characteristics of typical adakites, such as high SiO 2 (60.18–68.82 wt%) and Sr (499–793 ppm) contents, low Y (8.90–17.1 ppm) and Yb (0.88–1.58 ppm) contents, and high Sr/Y (26.1‒67.8) and (La/Yb) N (21.9‒32.9) ratios. They have variable K 2 O (3.88–5.09 wt%), MgO (0.44–2.74 wt%; Mg# = 33.7–52.5), Cr (4.27–40.59 ppm), Ni (4.28–35.68 ppm) and Th (9.56–59.59 ppm) contents, and relatively depleted Sr–Nd isotopic compositions [( 87 Sr/ 86 Sr) i = 0.70450–0.70516; ε Nd ( t ) = 2.1–2.7]. These characteristics indicate that the quartz monzonites were derived from the partial melting of delaminated lower crust that interacted with mantle peridotite with high MgO, Cr and Ni contents and depleted Sr–Nd isotopic compositions and suggest that the granites were formed by the fractional crystallization of quartz monzonitic magma. The geochemical features of the studied adakitic rocks could therefore have been affected by magmatic processes (e.g. fractional crystallization), which might be misleading in interpretations of their petrogenesis and related tectonic settings. The geochemical features of the studied rocks indicate that the crust of the western segment of the Urumieh–Dokhtar magmatic belt was thickened to c. 50 ± 4.43 km during the latest Oligocene ( c. 23.5 Ma) as a result of Arabia–Eurasia collision. Supplementary material: Whole rock geochemical and Sr–Nd isotopic compositions, and zircon U–Pb and Lu–Hf isotopic data are available at https://doi.org/10.6084/m9.figshare.c.6188328

Early Permian magmatism above a slab window in Inner Mongolia, North China: Implications for the Paleo-Asian Ocean subduction processes and accretionary crustal growth

Identifying magmatic rock associations in a subduction zone is substantial for understanding the related geodynamic evolution. The final closure time of the Paleo-Asian Ocean and the exact subduction processes have continuously been controversial, hindering our recognition of the tectonic evolution of the Central Asian Orogenic Belt (CAOB). Here we investigated the Early Permian gabbros-granodiorites from Xi Ujimqin in the northern Inner Mongolian region. The gabbros have slightly depleted light rare-earth elements [(La/Yb)N = 0.8] and flat heavy rare earth elements (HREE) [(Gd/Yb)N = 1.1] that are comparable to those of N-MORB. They show depletion in Nb and Ta, positive whole-rock εNd(t) values of +7.7 to +8.7 and zircon εHf(t) values of +8.7 to +10.6. These mafic rocks are interpreted to be products of partial melting of depleted mantle that had been metasomatized by slab-derived fluids, with subsequent fractional crystallization and minor crustal contamination. These granodiorites chemically resemble typical adakites and have MORB-like whole-rock SrNd isotopic compositions (87Sr/86Sri = 0.7028–0.7029; εNd(t) = +8.2–+8.5) and positive zircon εHf(t) (+13.4–+15.9) values, suggesting that they originated from the subducted slab-melts and reacted with mantle wedge peridotite. UPb zircon dating shows emplacement of N-MORB-type gabbros and adakitic granodiorites at ∼297 Ma and ∼290 Ma, respectively. Our new data indicate the presence coeval slab-derived adakites and slab fluid-metasomatized asthenosphere-derived N-MORB-type rocks, indicating that subduction lasted until at least the Early Permian. Such an association along with the extension-related magmatism in northern Mongolia recorded the northward ridge subduction of the Paleo-Asian Ocean during the Late Carboniferous–Early Permian. This model is consistent with the spatial distribution and the ages of magmatic activity with positive εNd(t)–εHf(t) values from this region. Ridge subduction and induced slab windows probably played a key role in Paleozoic crustal growth of CAOB, and by inference in the other accretionary orogens throughout Earth history.

Geochronology, geochemistry and tectonic implications of early Carboniferous plutons in the southwestern Alxa Block

AbstractThe southeastern Central Asian Orogenic Belt (CAOB) records the assembly process between several micro-continental blocks and the North China Craton (NCC), with the consumption of the Paleo-Asian Ocean (PAO), but whether the S-wards subduction of the PAO beneath the northern NCC was ongoing during Carboniferous–Permian time is still being debated. A key issue to resolve this controversy is whether the Carboniferous magmatism in the northern NCC was continental arc magmatism. The Alxa Block is the western segment of the northern NCC and contiguous to the southeastern CAOB, and their Carboniferous–Permian magmatism could have occurred in similar tectonic settings. In this contribution, new zircon U–Pb ages, elemental geochemistry and Sr–Nd isotopic analyses are presented for three early Carboniferous granitic plutons in the southwestern Alxa Block. Two newly identified aluminous A-type granites, an alkali-feldspar granite (331.6 ± 1.6 Ma) and a monzogranite (331.8 ± 1.7 Ma), exhibit juvenile and radiogenic Sr–Nd isotopic features, respectively. Although a granodiorite (326.2 ± 6.6 Ma) is characterized by high Sr/Y ratios (97.4–139.9), which is generally treated as an adikitic feature, this sample has highly radiogenic Sr–Nd isotopes and displays significantly higher K2O/Na2O ratios than typical adakites. These three granites were probably derived from the partial melting of Precambrian continental crustal sources heated by upwelling asthenosphere in lithospheric extensional setting. Regionally, both the Alxa Block and the southeastern CAOB are characterized by the formation of early Carboniferous extension-related magmatic rocks but lack coeval sedimentary deposits, suggesting a uniform lithospheric extensional setting rather than a simple continental arc.

Genesis of High-Mg Adakites in the Southeastern Margin of North China Craton: Geochemical and U-Pb Geochronological Perspectives

In the eastern North China Craton (NCC), Mesozoic tectonics was dominated by the Paleo-Pacific subduction rollback and the Tanlu crustal-scale fault movement. The regional transtension had generated extensive adakitic magmatism, some Cu-Au ore-forming but others not. To establish the geodynamic setting and any metallogenic link for the adakites from the southeastern NCC margin, we analyzed the ore-barren adakitic rocks from underground mines in the Huaibei-Linhuan coalfield (where surface igneous outcrops are scarce), and compared their ages and geochemistry with other mineralized and ore-barren adakites across Eastern China. Zircon U-Pb dating reveals two magmatic episodes in the Huaibei-Linhuan coalfield: 1) early-Early Cretaceous (ca. 130–129 Ma) (quartz-)diorite and granodiorite, and 2) late-Early Cretaceous (ca. 115.8 and 105.8 Ma) microgabbro and dolerite. Whole-rock geochemistry indicates that the (quartz-)diorite and granodiorite are high-Mg adakitic, featured by low K2O/Na2O (avg. 0.33), high Sr/La (avg. 44.3), and lack of correlation between SiO2 (fractionation index) and Sr/Y (avg. 56.55) and MREE/HREE (avg. 1.09), resembling typical adakites derived from oceanic-slab partial melting. Geochronological correlation with the regional tectonic events suggests that the slab-melting may have been caused by the Paleo-Pacific subduction rollback. Further extension and crustal thinning in the late-Early Cretaceous along the southern Tanlu fault may have formed the gabbro-dolerite in the coalfield. Geochemical comparison suggests that parental magma of the Huaibei-Linhuan adakites may have had similar water content [similar zircon 10,000*(Eu/Eu*)/Y and Eu/Eu* ratios] to typical porphyry Cu-Au ore-forming magmas, yet the former may have been considerably more reduced (lower zircon Ce/Nd and whole-rock V/Sc ratios). We considered that the assimilation of Carboniferous-Permian coal seams in the area may have further lowered the magma fO2 and thus its potential to form Cu-Au mineralization.

Adakites and associated granitoids from the Serra da Prata Arc: Evidence for a Tonian subduction setting within the Araçuaí-Ribeira orogenic system (AROS), SE Brazil

Within deeply eroded orogenic belts, the geochemical signatures of metamorphosed igneous rocks can be used to constrain modern-like tectonic models operating in Precambrian times. One important aspect of this discussion is the presence or not of pre-collisional arc-related rocks that would represent the magmatic product of subduction processes imprinted on the upper plate of the convergent setting. New detailed geological and geochemical data of the northern segment of the Tonian to Cryogenian juvenile Outer Arc of the Araçuaí-Ribeira Orogenic System (AROS) are presented. Results were combined with recently published U–Pb and Hf data to contribute to a comprehensive tectonic model. The arc-related rocks (Serra da Prata Complex), transformed in foliated orthogneisses, are interlayered with dolomitic marbles, amphibolites, and psammo-pelitic paragneisses (Italva Group). This association is similar to the one described in the southern part of the orogenic system, pointing to the extension of the Outer Arc system until the Rio Doce River, in the transition zone between the Araçuaí and Ribeira segments of the AROS. New geochemical analyses together with reinterpreted published data indicate the presence of typical adakites that, together with normal calc-alkaline expanded series, island arc tholeiitic series, and A-type granitoids, corroborate with a modern style plate tectonic model. The envisaged model started in the Tonian, with E-verging subduction that resulted in the production of very juvenile arc-related rocks and associated fore-to back-arc basins filled with carbonatic rocks, meta-volcanic rocks, and MORB-like basaltic rocks. The typical adakites are considered the result of partial melting of young and hot oceanic lithosphere under moderate to high pressures in which garnet was an important stable residue. Potassic C-type adakitic rocks formed by the partial melting of thickened lower crust occur associated with the typical adakites. The occurrence of both adakites, together with A-type granitoids, characteristic of the investigated terminal sector of the Serra da Prata arc, is interpreted to result from a slab window gap, either associated with slab tearing/ridge subduction or slab roll-back, hampering an intracontinental model for the Ribeira orogen.

Late Paleoproterozoic crustal thickening of the Jiao–Liao–Ji belt, North China Craton: Insights from ca. 1.95–1.88 Ga syn-collisional adakitic granites

The Jiao–Liao–Ji belt (JLJB) of the North China Craton is commonly regarded to have been involved in the global-scale collisional events that led to the amalgamation of the Columbia/Nuna supercontinent during the late Paleoproterozoic. Previous studies have revealed that the JLJB underwent a high-pressure granulite-facies metamorphic event with a clockwise P–T–t path during ca. 1.95–1.90 Ga. However, few studies have examined the contemporaneous magmatism and its genetic relationship to the high-pressure granulite-facies metamorphism. In addition, the petrogenesis of the ca. 1.95–1.90 Ga magmatism and its possible constraints on the crustal-thickening process are poorly understood. Our field investigations have discovered a series of late Paleoproterozoic granitoid plutons in Liaodong Peninsula, including the Fangjiawei, Qingchengzi, Songshugou, and Shizijie plutons. Systematic analyses of geochronology, petrology, and geochemistry were conducted to constrain the formation ages, petrogenesis, and tectonic environment of these granites. Zircon U–Pb dating reveals that the granites formed during ca. 1.95–1.88 Ga, coeval with the timing of the regional prograde or peak metamorphism and collision in the JLJB. They show geochemical characteristics of typical adakites, including high Sr (393–860 ppm) contents, low Y (1.03–3.30 ppm) and Yb (0.09–0.27 ppm) contents, and relatively high Sr/Y (125–832) and LaN/YbN (9.34–33.6) ratios. Low MgO, Cr, and Ni contents and the existence of ca. 2.5–2.0 Ga inherited zircons with ancient TDM2 ages indicate that the parental magmas were generated from partial melting of a thickened lower crust. The positive correlation between Dy/Yb and La/Yb ratios and the relatively low Nb/Ta (5.1–18.9, average of 13.6) ratios all suggest that garnets, rather than amphiboles or rutiles, were the dominant phase in the residual source, indicating a melting depth of 30–50 km. Our new results, combined with previous findings, support a rifting-and-collision model whereby the JLJB underwent a transformation from early intracontinental rifting to subsequent opening and closure of an incipient ocean, with the final collision between the Longgang Block and Nangrim Block occurring at ca. 1.95–1.88 Ga with substantial crustal thickening.

Mineralogy Constraints on Magmatic Processes Controlling Adakitic Features of Early Permian High-magnesium Diorites in the Western Tianshan Orogenic Belt

AbstractWhole-rock geochemistry, usually changed by magmatic processes, might provide misleading information on the petrogenesis of adakites. The Heishantou porphyritic diorites in Nileke, Western Tianshan orogenic belt record complex magma chamber processes, such as magma replenishment, fractional crystallization and crustal contamination, and thus, provide ideal samples for tracing the magmatic processes that generate the typically high Sr and La contents and Sr/Y and La/Yb ratios of adakites. In situ clinopyroxene and amphibole compositions of Heishantou porphyritic diorites (271 ± 2 Ma) are characterized by low Yb and Y content with high Sr/Y and La/Yb ratios, from which calculated early magmas resemble typical adakites derived from partial melting of a subducted slab. But whole-rock composition shows low MgO, Ni and Cr, thus, the Heishantou diorites were previously regarded as the result of partial melting of thickened lower crust. Plagioclase phenocrysts exhibit complex compositional zoning due to magma replenishment, and the rims have higher 87Sr/86Sr ratio and Sr content than the cores, indicating crustal assimilation. The cores of zoned clinopyroxene phenocrysts have high Mg#, Cr and Ni but low Yb and Y with high Sr/Y and La/Yb ratios, which are consistent with the high Mg# of primary adakitic magmas. Magmatic processes have significantly changed the primary adakitic features of Yb, Y and Sr content, Sr/Y and La/Yb ratios, in addition to Mg# values. The Heishantou primitive high-Mg# adakite was derived from partial melting of a delaminated lower crust followed by storage, recharge, and assimilation in a crustal magma chamber. The Western Tianshan orogenic belt experienced a succession of lower crust delamination events in the Early Permian that involved melting of thickened lower crust, subsequent lithospheric detachment and asthenospheric upwelling.

Late Miocene to Pliocene crustal extension and lithospheric delamination revealed from the ~5 Ma Palopo granodioritic intrusion in Western Sulawesi, Indonesia

Recent studies have revealed that following collision between the Western Sulawesi and Sula Spur microcontinents during the early to middle Miocene, the Western Sulawesi microcontinent subsequently underwent extension. However, the geodynamic process that resulted in extension is unknown. The petrogenesis of late Miocene to Pliocene granitoids in the region provides insights to the mechanism of extension. To achieve this goal, we studied the Palopo granodioritic intrusion, a well-exposed representative pluton that crops out along the Palu–Koro Fault. Zircon U–Pb dating indicates a 4.8–4.7 ± 0.1 Ma crystallization age for the pluton. Compared with arc igneous rocks, the pluton has high Sr contents and low Y and Yb contents, with relatively high Sr/Y and LaN/YbN ratios. These geochemical signatures are similar to those of typical adakites. The samples exhibit high MgO (1.60–2.54 wt%), Cr (46–65 ppm), and Ni (14.1–19.0 ppm) contents relative to adakitic rocks derived from thickened lower crust. In addition, negative εHf(t) (−9.7 to −5.5) and εNd(t) (−8.5 to −11.1) values, together with relatively low 143Nd/144Nd and high initial 87Sr/86Sr ratios, reflect an origin through recycling of ancient crustal materials, which is clearly different from adakites derived from melting of a subducting slab. The most plausible mechanism for generation of the late Miocene–Pliocene adakitic granodiorites in the Western Sulawesi microcontinent is partial melting of delaminated lower crust that interacted with the mantle. We propose that the late Miocene to Pliocene crustal extension in the Western Sulawesi microcontinent was triggered by delamination of pre-existing thickened lithosphere.

Geochronological and geochemical studies of adakites from Tethyan Belt, Western Pakistan: A clue to geodynamics and Cu-Au mineralization

ABSTRACT Large porphyry Cu-Au deposits are associated with Early Miocene intrusive rocks in Tethyan belt, discovered along Chagai magmatic arc in Western Pakistan, adjacent to Southeast Iran. Two types of rocks were discriminated as granodiorite and monzodiorite from Saindak area. The granodiorites are associated with regional large Cu-Au mineralization, while the monzodiorites are mostly ore-barren. LA-ICP-MS U-Pb dating yielded weighted mean 206Pb/238U ages of 22.16–24.2 Ma for granodiorites and ca. 22.6 Ma for a monzodiorite. These intrusive rocks in the Saindak area are all calc-alkaline rocks, with enriched LILEs and depleted HFSEs, and without Eu negative anomalies. The felsic granodiorites are characterized by high Sr contents and Sr/Y ratios, with intermediate to high (La/Yb)N ratios that identify them as typical high-silica adakites. In contrast, the basaltic-andesitic monzodiorites are just normal arc-related rocks, showing less fractionated REE patterns than the granodiorites, with systematically lower LREE and higher MREE and HREE. Low K2O/Na2O ratios and decoupled Sr/Y-(La/Yb)N ratios indicate the characteristics of slab-derived adakites for the granodiorites, most likely originated through partial melting of the subducted Neotethys oceanic crust beneath Eurasian continent followed by subsequent mantle interaction. The almost simultaneously melting of subcontinental lithospheric mantle induced by dehydration of Neotethys plate gave rise to the formation of the barren monzodiorites. These intrusive rocks in the Saindak area are characterized by similar Sr-Nd-Pb isotopes with high 87Sr/86Sri ratios, slightly negative εNd(t) values and radiogenic Pb isotopes, plotting in the field between the MORB and EM-II mantle endmembers or the Average Cadomian Lower Crust, suggesting subducting sediments or old continental crustal materials have contributed into the compositions of these rocks by source enrichment or crustal contamination. The slab-melting derived adakite is favourable for regional massive Cu-Au mineralization in the Saindak area.

Geochronology and geochemistry of the Sikongshan intrusion in the Dabie Orogen, Central China: Implication for Mesozoic geodynamic background

The Sikongshan (SKS) intrusion, the largest pluton situated in the Central Dabie unit of the Dabie Orogen (DBO), is composed of monzogranite and quartz diorite. A study combining whole‐rock major and trace element analysis, zircon U–Pb dating, as well as zircon in situ Lu–Hf isotopes was conducted for the SKS intrusion. Zircon dating yielded U–Pb ages of 136 ± 1.6 Ma and 134.6 ± 1.7 to 130.4 ± 1.7 Ma for quartz diorites and monzogranites, respectively, both types belonging to the early stage of the Mesozoic magmatism in the DBO. The quartz diorites exhibited high Na2O (Na2O/K2O = 1.60–1.86) and Sr (1,185–1,365 ppm) and low Y (11.6–14.2 ppm) contents, and therefore high Sr/Y (83.45–108.19) ratios. They also had high ratios of (Dy/Yb)N (48.42–54.91) and (Dy/Yb)N (1.47–1.53). The monzogranites were enriched in K2O (Na2O/K2O = 0.64–0.99) and were also characterized by low Y (5.6–14.7 ppm) contents and high Sr/Y ratios with a geochemical affinity similar to that of the adakites. However, the Sr (<358 ppm) and Al2O3 (3.31–14.92%) contents were much lower than those in typical adakites and quartz diorites. Both the quartz diorites and monzogranites had trace element signatures similar to that of continental crust with enriched levels of Rb, Ba, Th, U, and LREE and depleted levels of Nb, Ta, P, and Ti, indicating their crustal origin. Following an integrated review of these and other data from previous studies, it was concluded that the Sikongshan quartz diorite originated from the thickened lower crust of the DBO, with a composition similar to that of the Huangtuling granulite, which is the exposed lower crust of the DBO. The monzogranite was generated by plagioclase + amphibolite fractional crystallization of the adakitic Tiantangzhai melt. The adakitic rocks in the DBO originated in the western part during 143 to 134 Ma and then spread extensively throughout the whole orogen (134 to 130 Ma). The collapse of the thickened mountain root in the DBO at an extensional setting may be related to the roll‐back of the subducted Paleo‐Pacific slab.

Petrogenesis of the late Early Palaeozoic adakitic granitoids in the southern margin of the Songliao Basin, NE China: Implications for the subduction of the Palaeo‐Asian Ocean

Many data have been reported from the Early Palaeozoic subduction complex belt and accretionary arc to the south of the Solonker‐Xar Moron‐Changchun‐Yanji Suture (SXCYS) zone. However, as the suture is covered to the east by the Mesozoic–Cenozoic Songliao Basin, there have yet been no related reports about the Early Palaeozoic igneous rocks in this area. The authors discovered an Early Palaeozoic granitoid intrusion in the Fangjiatun area, northern Liaoning, and performed petrologic, geochemical, zircon U–Pb dating and Hf isotopic analyses on it. Zircon SHRIMP U–Pb dating results show that the intrusion formed at 437–432 Ma, that is, in the late stage of the Early Silurian. Petrogeochemical analysis indicates that the intrusion records 63.14%–66.45% SiO2, with Na2O/K2O ratios of 1.01–6.12 and magnesian index (Mg#) values of 32.42–40.82. The REEs show a certain degree of LREE/HREE fractionation and slightly lower ΣREE contents. The trace elements are relatively enriched in Rb, Ba, Th, K, and Sr (large‐ion lithophile elements [LILEs]) and depleted in Nb, Ta, P, and Ti (high‐field‐strength elements [HFSEs]), displaying continental magmatic arc characteristics. The rocks have high Sr mass fractions, low Y and Yb mass fractions, and high Sr/Y ratios, which are consistent with the geochemical signatures of typical adakites. Zircon Hf isotopic analyses indicate that their εHf(t) values range from 0.29–6.56, reflecting the relatively depleted signatures of the source area. Based on its occurrence location and emplacement time, the intrusion is interpreted to have been produced by tectonic thermal events during the subduction of the Palaeo‐Asian oceanic crust beneath the northern margin of the North China Craton (NCC), thus reflecting the existence of an Early Palaeozoic continental marginal accretionary arc in the eastern segment of the NCC along the south of the SXCYS and provide new evidence for the tectonic evolution of the northern margin of the NCC in the Early Palaeozoic.

The genetic relationship between JTA–like magmas and typical adakites: An example from the Late Cretaceous Nuri complex, southern Tibet

Phanerozoic adakites typically represent modern analogues of Middle–Late Archean (3.5–2.5 Ga) tonalite–trondhjemite–granodiorite rocks (TTGs) due to their similarities in geochemical compositions. However, no Phanerozoic analogues have been reported for the Early Archean TTGs (>3.5 Ga), which are characterized by low Sr, Y and HREEs contents with low MgO and compatible elements contents, until recent years. The newfound Phanerozoic analogues have been defined as a new adakite subgroup named as “Jamaican–type adakites” (JTAs), and been regarded as the results of partial melting of subducted oceanic plateau crust. JTA–like magmas also occur in the Nuri complex, southern Tibet. We report whole–rock geochemical, SrNd isotopic data, zircon UPb ages and in-situ Hf isotopic data for adakitic quartz diorites, quartz monzonites and JTA–like quartz porphyries in the Nuri complex. Zircon UPb dating indicates that these rocks were emplaced in the Late Cretaceous at 93.5–92.1 Ma. Combined with the continuous variation trends of major and trace elements, the similarities in terms of whole–rock SrNd and zircon Hf isotopic compositions indicate that the three types of rocks are genetically related, while the quartz diorites should represent the relatively primitive melts. High MgO (5.00–5.41 wt%), Cr (138–159 ppm), Ni (65.9–96.8 ppm) contents and Mg# values (66–69), high εHf(t) values (10.5–11.9), εNd(t) values (3.7), and low (87Sr/86Sr)i values (0.7046–0.7050) indicate that the quartz diorites were derived from partial melting of subducted Neo–Tethyan oceanic slab, and subsequently underwent metasomatic reaction with the mantle wedge during their ascent. Quartz monzonites and JTA–like quartz porphyries were originated from quartz diorites by two stages of fractional crystallization. Low MgO and compatible elements contents of the quartz porphyries were caused by fractionation of hornblende, biotite, with minor magnetite and allanite, while low Sr contents and Sr/Y ratios were mainly controlled by plagioclase fractionation. This means that the JTA–like geochemical features of the quartz porphyries were generated by fractional crystallization from the adakites derived from partial melting of subducted slab, rather than directly derived from partial melting of subducted oceanic plateau. Therefore, various petrogenetic possibilities of the JTA–like magmas should be considered when using them as modern analogues.

Petrogenesis of the Pulang porphyry complex, southwestern China: Implications for porphyry copper metallogenesis and subduction of the Paleo-Tethys Oceanic lithosphere

The Pulang complex is located in the southern segment of the Yidun Arc in the Sanjiang Tethys belt, southwestern China. It is composed of quartz diorite, quartz monzonite and granodiorite porphyries, and hosts the super-large Pulang deposit. This study presents new U-Pb geochronological, major-trace elemental and Sr-Nd-Hf isotopic data to constrain the petrogenesis of the Pulang complex and to evaluate its significances for porphyric mineralization and tectonic evolution of the Paleo-Tethys Ocean. The zircon U-Pb dating yields ages ranging from 208Ma to 214Ma. Geochemically, the Pulang complex has high Sr and MgO contents, and high Sr/Y and La/Yb ratios, but low Yb and Y contents, displaying adakitic affinities. However, it has moderate Sr/Y and La/Yb ratios, and high Rb contents (32 to 202ppm). The Pulang samples plot into the transitional field between adakites and normal arc rocks, differing from typical adakites. It is attributed to the assimilation of 10–15% crustal components. The zircon εHf(t) (−4.6 to −2.5), whole-rock (87Sr/86Sr)i (0.7052 to 0.7102), εNd(t) (−0.62 to 2.12) values and adakitic affinities suggest that the Pulang complex was derived from a basaltic slab-melt source and reacted with peridotite during ascending through an enriched asthenospheric mantle wedge. The basaltic slab-melts likely resulted from the westward subduction of the Ganzi-Litang oceanic plate (a branch of the Paleo-Tethys). As far as the metallogenesis concerned, three factors in mineralization are proposed in this paper. The country rock, quartz diorite porphyry, has higher Cu contents than the mantle (average 30ppm), suggesting that ore-forming magma was derived from a Cu-enriched source, which is a crucial contribution to the late mineralization to form the super-large Pulang deposit. In addition, the barren quartz diorite, granodiorite, and ore-bearing quartz monzonite porphyries are all characterized by high oxygen fugacity, which is another important factor for the mineralization. Moreover, the separation/exsolution of the quartz monzonite from the quartz diorite is vital to the enrichment of Cu in the Pulang deposit. The emplacement of the Pulang pluton, posterior to the closure of the Jiangshajiang Ocean (a branch of the paleo-Tethys), is consistent with the igneous rocks associated with the westward subduction of the Ganzi-Litang oceanic plate during late Triassic Time. The Zhongdian Arc in the southern segment of the Yidun Arc was close to the arc front that was conducive to Cu mineralization in the porphyry system.

Petrology of calc-alkaline/adakitic basement hosting A-type Neoproterozoic granites of the Malani igneous suite in Nagar Parkar, SE Sindh, Pakistan

The Nagar Parkar area contains three distinct groups of rocks, from oldest to youngest, (1) basement rocks ranging in composition from mafic to (quartz)diorite, tonalite, granite, and younger granodiorite, (2) granite plutons similar in general features to those of the Malani Igneous Suite of Rajasthan, and (3) abundant mafic, felsic and rhyolitic dykes. The basement rocks show strong brittle and local plastic deformation, and epidote amphibolite/upper greenschist facies metamorphic overprint. The chemistry of the basement rocks contrasts the commonly agreed within plate A-type character of the Neoproterozoic granites (group 2) that are emplaced into them. The basement rock association is calc-alkaline; the granodiorite displays the compositional characteristics of adakites, whereas the tonalite has intermediate composition between typical adakite and classical island arc rocks. This paper presents detailed petrography of the basement rocks and compares their geochemistry with those of the group 2 granites as well as with rocks from other tectonic environments. It is proposed that the Nagar Parkar basement is part of a 900–840 Ma magmatic arc that was deformed before it was intruded 800–700 Ma ago by the A-type continental granitic rocks followed by mafic to felsic dykes.

Carboniferous porphyry Cu–Au deposits in the Almalyk orefield, Uzbekistan: the Sarycheku and Kalmakyr examples

ABSTRACTThe Almalyk porphyry cluster in the western part of the Central Asian Orogenic Belt is the second largest porphyry region in Asia and hence has attracted considerable attention of the geologists. In this contribution, we report the zircon U–Pb ages, major and trace element geochemistry as well as Sr–Nd isotopic data for the ore-related porphyries of the Sarycheku and Kalmakyr deposits. The zircon U–Pb ages (Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)) of ore-bearing quartz monzonite and granodiorite porphyries from the Kalmakyr deposit are 326.1 ± 3.4 and 315.2 ± 2.8 Ma, and those for the ore-bearing granodiorite porphyries and monzonite dike from the Sarycheku deposit are 337.8 ± 3.1 and 313.2 ± 2.5 Ma, respectively. Together with the previous ages, they confine multi-phase intrusions from 337 to 306 Ma for the Almalyk ore cluster. Geochemically, all samples belong to shoshonitic series and are enriched in large-ion lithophile elements relative to high field strength elements with very low Nb/U weight ratios (0.83–2.56). They show initial (87Sr/86Sr)i ratios of 0.7059–0.7068 for Kalmakyr and 0.7067–0.7072 for Sarycheku and low εNd(t) values of −1.0 to −0.1 for Kalmakyr and −2.3 to 0.2 for Sarycheku, suggesting that the magmas were dominantly derived from a metasomatized mantle wedge modified by slab-derived fluids with the contribution of the continental crust by assimilation-fractional-crystallization process. Compared to the typical porphyry Cu deposits, the ore-bearing porphyries in the Almalyk cluster are shoshonitic instead of the calc-alkaline. Moreover, although the magmatic events were genetically related to a continental arc environment, the ore-bearing porphyries at Sarycheku and Kalmakyr do not show geochemical signatures of typical adakites as reflected in some giant porphyry deposits in the Circum-Pacific Ocean, indicating that slab-melting may not have been involved in their petrogenesis.

A Silurian-early Devonian slab window in the southern Central Asian Orogenic Belt: Evidence from high-Mg diorites, adakites and granitoids in the western Central Beishan region, NW China

The Beishan orogenic belt is a key region for deciphering the accretionary processes of the southern Central Asian Orogenic Belt. Here in this paper we present new zircon U–Pb ages, bulk–rock major and trace element, and zircon Hf isotopic data for the Baitoushan, and Bagelengtai plutons in the western Central Beishan region to address the accretionary processes.The Baitoushan pluton consists of quartz diorites, monzonites and K-feldspar granites, with zircon LA–ICP–MS U–Pb ages of 435Ma, 421Ma and 401Ma, respectively. The Baitoushan quartz diorites and quartz monzonites exhibit relatively high MgO contents and Mg# values (63–72), display enrichments in LILEs and LREEs, and exhibit high Ba (585–1415ppm), Sr (416–570ppm) and compatible element (such as Cr and Ni) abundances, which make them akin to typical high-Mg andesites. The Baitoushan quartz diorites and quartz monzonites were probably generated by the interaction of subducted oceanic sediment-derived melts and mantle peridotites. The Baitoushan K-feldspar granites are ascribed to fractionated I-type granites with peraluminous and high-K calc-alkaline characteristics. They exhibit positive εHf(t) values (2.43–7.63) and Mesoproterozoic–Neoproterozoic zircon Hf model ages (0.92–1.60Ga). Those early Devonian granites, including Baitoushan K-feldspar granite and Gongpoquan leucogranites (402Ma), are derived from melting of the mafic lower crust and/or sediments by upwelling of hot asthenospheric mantle. The Bagelengtai granodiorites exhibit similar geochemical signatures with that of typical adakites, with a zircon SHRIMP U-Pb age of 435Ma. They exhibit relatively high Sr (502–628ppm) and Al2O3 (16.40–17.40wt.%) contents, and low MgO (1.02–1.29wt.%), Y (3.37–6.94ppm) and HREEs contents, with relatively high Sr/Y and (La/Yb)N ratios. The Bagelengtai granodiorites were derived from partial melting of subducted young oceanic crust, with significant contributions of subducted sediments, subsequently hybridized by the mantle peridotite.On the basis of our data and combined with previous data, we conclude that a Silurian–early Devonian slab window model accounts for all geological records in the western segment of the Central Beishan. We further propose that magmatic events associated with a slab window played an important role in crustal growth of the Central Asian Orogenic Belt.

B isotopes of Carboniferous‐Permian volcanic rocks in the Tuha basin mirror a transition from subduction to intraplate setting in Central Asian Orogenic Belt

AbstractControversies remain as to the Permian tectonic setting in the Central Asian Orogenic Belt (CAOB), in particular, regarding the triggering of Permian magmatism. To address this issue, we carried out a geochemical study on Dananhu volcanics from the Tuha basin, southwestern CAOB. 40Ar/39Ar analyses for feldspar separates yield a Carboniferous age of 321.2 ± 9.8 Ma for the andesites and a Permian age of 278.9 ± 4.2 Ma for the basalts. Both the andesites and basalts geochemically resemble subduction‐related magmas in aspects of their trace element and Sr‐Nd‐Pb isotopic composition, but they may differ in petrogenesis. The andesites are typical adakites. Their high Cr and Ni contents, and low heavy rare earth element contents, and positive correlation between B/Nb ratios and δ11B values suggest that they were generated by interaction between slab‐derived melts/fluids and the mantle wedge. The Permian basalts are subdivided into two subgroups: alkali and tholeiitic basalts. High Ba/La and Ba/Zr ratios indicate that the source of the alkali basalts has been metasomatized by fluids/melts derived from altered oceanic crust. In contrast, a volatile‐free source metasomatized by sediment‐derived fluids/melts is inferred for the tholeiitic basalts given their high Th/Zr and Th/Ce ratios. Negative correlation between B/Nb and δ11B for the Permian basalts imply that the arc signatures were imposed by previous subduction events, and no direct slab‐derived fluids/melts participated in their petrogenesis. These geochemical characteristics alongside regional geologic records collectively suggest a tectonic shift in the southwestern CAOB from subduction‐related setting during the Late Carboniferous to an intraplate setting during the Early Permian.