Petrogenesis of Permian‐Triassic Sanukites in the Funing Area, South China: Insights into Backarc Basin Magmatism Driven by Paleo‐Tethys Plate Subduction

The mafic igneous activities in the late Triassic play an important role in the tectonic evolution of the South China Block (SCB). In this paper, relevant data of the bojites in Yajiangqiao area, eastern Hunan Province, South China were provided, including zircon U–Pb ages, whole‐rock geochemical data, Sr–Nd–Pb isotopic data, and zircon Hf isotope data. These data indicate that the bojites were crystallized at ~ 215.7 Ma, and thus are coeval with biotite monzonitic granites of Yajiangqiao pluton. The bojite samples feature a low SiO2 content of 48.84–49.94 wt%, a low ALK (K2O + Na2O) content of 5.13–6.13 wt%, a relatively high Al2O3 content of 17.64–21.22 wt%, a moderate MgO content of 4.32–7.07 wt%, and a moderate FeOT content of 5.66–7.33 wt%. In addition, the samples have a total rare earth element (REE) content of 108–163 ppm. They display right‐declined REE patterns, enriched in light rare earth elements and large‐ion lithophile elements (such as Rb, K, Th, and U) and depleted in high field strength elements (such as Nb, Ta, P, and Ti). All the samples have homogeneous Sr–Nd–Pb and zircon Hf isotopic compositions, with the (87Sr/86Sr)i ratio varying from 0.708 202 to 0.709 515, (143Nd/144Nd)i ratio from 0.512 156 to 0.512 229, (206Pb/204Pb)t ratio from 18.185 to 18.264, (207Pb/204Pb)t ratio from 15.667 to 15.672, (208Pb/204Pb)t ratio from 38.510 to 38.587, and initial 176Hf/177Hf ratio ranging from 0.282 428 to 0.282 530. Meanwhile, the calculated εNd(t) and εHf(t) values range from −4.0 to −2.6 and from −7.9 to −4.1, respectively, with two‐stage Nd model ages of 1.20–1.32 Ga and two‐stage Hf model ages of 1.35–1.56 Ga, respectively. As indicated by geochemical data, the primitive magmas of the bojites in Yajiangqiao area were derived from lithospheric mantle in post‐collisional extensional environment, which underwent low‐degree partial melting (1–5 %) of spinel‐garnet lherzolites (spinel > garnet) and was subjected to metasomatism of slab‐derived fluids. Furthermore, it can be concluded from these data and previous data that the SCB was transformed from syn‐collisional compression to post‐collisional extension in the late Triassic (~ 234 Ma) and the post‐collisional extension lasted until 215 Ma.

We present an integrated study involving detailed field investigation, petrography, whole-rock geochemistry, Sr–Nd isotopic compositions, and zircon U–Pb–Hf isotopic compositions of Late Cretaceous granitoids and gabbroic diorites in the Shanba and Zongga plutons of the southern Lhasa subterrane, Tibet. We use the obtained data to assess the genetic relationship between the granitoids and gabbroic diorites and to better constrain the origin of Late Cretaceous granitoids in the Gangdese batholith. Both the Shanba and Zongga plutons contain Late Cretaceous gabbroic diorites with zircon U–Pb ages of 91–90 Ma, but only the Zongga pluton includes coeval Late Cretaceous (88–83 Ma) granitoids (tonalites and granodiorites) that coexist with the gabbroic diorites. The Shanba gabbroic diorites consist mainly of clinopyroxene, biotite, and plagioclase, whereas the Zongga gabbroic diorites are composed predominantly of amphibole and plagioclase. The Late Cretaceous granitoids and gabbroic diorites in the Shanba and Zongga plutons are enriched in large-ion lithophile elements and light rare earth elements (REEs), and are depleted in high-field-strength elements and heavy REEs, showing an arc-type geochemical signature. The Zongga gabbroic diorites have lower K2O and light REE contents and Th/Y ratios, as well as higher Ba/La, Ba/Nb, and Rb/Nb ratios, compared with the Shanba gabbroic diorites. The Zongga gabbroic diorites and granitoids have similar Sr–Nd–Hf isotopic compositions [ISr = 0.7035–0.7042, eNd(t) = 5.0–6.4, eHf(t) = 9.6–13.8], with more radiogenic Nd isotopic compositions than those of the Shanba gabbroic diorites [ISr = 0.7037–0.7040, eNd(t) = 3.1–4.1, eHf(t) = 10.9–14.1]. These geochemical and isotopic data indicate that the Zongga granitoids are genetically related to their spatially and temporally associated Zongga gabbroic diorites, whose mantle source was more hydrous than that of the Shanba gabbroic diorites. On the basis of the petrological, geochemical, and isotopic data, we propose that the Shanba and Zongga gabbroic diorites were generated by partial melting of a depleted mantle wedge that was metasomatized by slab sediment melts and by slab dehydration fluids, respectively. The Zongga granitoids were produced by dehydration melting of juvenile lower crust compositionally similar to the Zongga gabbroic diorites, and the parental magma of these gabbroic diorites was also involved in their generation. We conclude that Late Cretaceous underplating of mantle-derived magmas with differing water contents formed juvenile lower crust in the southern Lhasa subterrane; however, only the more hydrous juvenile lower crust was able to produce voluminous granitoids through dehydration melting. Hydrous mantle-derived magmas, therefore, played a key role in the generation of Late Cretaceous granitoids in the Gangdese batholith and contributed not only to crustal growth but also to crustal reworking in the southern Lhasa subterrane.

Although high-Mg andesites (HMA) have attracted increasing attention due to their unique geochemical composition and important geological significance, there is no consensus on their petrogenesis. The present study indicates that the subducting terrigenous sediment-derived hydrous melts were incorporated into the mantle source of Paleozoic HMA in northwestern Tianshan, western China. These HMA are composed of basaltic andesite and andesite. They generally exhibit arc-type trace element distribution patterns and weakly enriched Sr-Nd-Hf isotope compositions. Some of them show remarkably higher Ba/La, Ba/Th, Rb/Nb, and U/Th ratios than normal mid-ocean ridge basalts (MORB). Furthermore, most of them are characterized by higher Th/Nb, Th/Yb, and Th/Nd ratios but lower Nb/U ratios relative to normal MORB, similar to those of terrigenous sediments. Whole-rock Nd isotopes covary with Rb/Nb and Th/Yb ratios, indicating contributions from both oceanic crust-derived aqueous solutions and terrigenous sediment-derived hydrous melts. Together with their high zircon δ18O values, it appears that the mantle source of the target HMA contains terrigenous sediment-derived hydrous melts in addition to subducting oceanic crust-derived aqueous solutions. In the studied HMA, the andesite generally exhibits higher contents of large-ion lithophile elements and light rare earth elements and more enrichments in Sr-Nd-Hf isotopes than the basaltic andesite. These differences indicate that the mantle source of andesite would contain more subducting sedimentderived hydrous melts than that of basaltic andesite. As a consequence, relatively Si-rich and Si-poor pyroxenite sources were respectively generated as the mantle sources of the andesite and basaltic andesite. This qualitative interpretation is verified by quantitative modeling of the geochemical transfer from subducting oceanic crust into the mantle wedge. Model calculations indicate that the addition of ~3% oceanic crust-derived aqueous solutions and 4%–12% terrigenous sediment-derived hydrous melts into the mantle wedge peridotite can account for the geochemical compositions of the target HMA. Therefore, the HMA in northwestern Tianshan provide the geochemical evidence for the crust-mantle interaction during the oceanic subduction in the Paleozoic. As such, the subducting terrigenous sediment-derived hydrous melts play a dominant role in the composition of the lithochemically fertile, geochemically enriched mantle sources and thus in the origin of HMA above oceanic subduction zones.

To better reveal the tectonic process of the Jiangnan Orogen in creating the united South China Block (SCB), a set of new geochronological, geochemical, and Sr-Nd-Hf-O isotopic data is herein presented for the coeval early Neoproterozoic mafic to acid rocks in the Lengjiaxi Group and its equivalents. Our data show that these rocks belong to subalkaline basalt, basaltic andesite, andesite, and dacite in composition, and can be geochemically classified into N-MORB- and arc-like mafic (Group 1 and Group 2, respectively), along with high-Mg andesitic-dacitic (Group 3) rocks. They yield zircon U-Pb ages of 837 ± 4 Ma, 832 ± 11 Ma, and 835 ± 7 Ma, respectively, suggesting their early Neoproterozoic (ca. 835 Ma) origin. Group 1 has SiO2 = 47.21–52.45 wt%, MgO = 7.36–10.07 wt% with Mg-number of 56–68, 87Sr/86Sr(t) = 0.7062–0.7136 and εNd(t) = +1.3–+9.4, along with N-MORB-like REE- (rare earth element) and PM (primitive mantle)-normalized patterns, suggesting its derivation from a refractory MORB-like source newly introduced by slab-derived fluid. The Group 2 samples are classified as tholeiitic basalt and basaltic andesite with SiO2 = 48.78–55.75 wt%, MgO = 7.70–12.74 wt%, Mg-number = 63–77, 87Sr/86Sr(t) = 0.7046–0.7098 and εNd(t) = –1.6 to –0.4. They exhibit similar “spiky” REE- and PM-normalized patterns to typical arc basalt, with pronounced enrichment in (large ion lithophile elements) and sharp depletion in (high field strength elements), probably inheriting from a sub-arc source modified by fluids and/or melts released from recycled sedimentary component. The Group 3 rocks show SiO2 = 57.74–66.30 wt%, FeOt = 6.22–8.59 wt%, Mg-number = 43–66, K2O+Na2O = 3.18–6.17 wt% with K2O/Na2O of 0.59–3.28, classified as high-Mg andesites and dacites. Diagnostic elemental and isotopic ratios of Group 3 include (La/Yb)cn = 7.7–11.4, (Gd/Yb)cn = 1.44–1.87, Eu/Eu* = 0.49–0.80, Nb/La = 0.27–0.42 and εNd(t) = –7.3 to –5.4, resembling to those of other early Neoproterozoic high-Mg intermediate rocks in the SCB, originating from a mantle wedge source with proportional addition of the recycled SCB sediment-derived components. In combination with the available data and geological observations, it is concluded that the N-MORB- and arc-like mafic, along with high-Mg andesitic rocks are coeval in the Lengjiaxi Group and its equivalents along the central Jiangnan Orogen and formed at ca. 847–832 Ma of the early Neoproterozoic period. The synthesis of these observations suggests the development of an early Neoproterozoic continental arc-basin setting. The arc-basin system westerly subducted and finally closed at ca. 830 Ma and the assembly of the Yangtze and Cathaysia blocks terminated at ca. 810 Ma along the central Jiangnan Orogen.

Petrogenesis of basalt–high-Mg andesite–adakite in the Neoarchean Veligallu greenstone terrane: Geochemical evidence for a rifted back-arc crust in the eastern Dharwar craton, India

Geochemistry and Sr–Nd–Hf–Pb isotope systematics of late Carboniferous sanukitoids in northern West Junggar, NW China: Implications for initiation of ridge-subduction

Author index to volume 58

Subduction retreating caused the external breakup of Rodinia: Constraints from the Neoproterozoic igneous rocks in the western Yangtze Block, South China

The Cambrian period marks a crucial phase in the initial subduction of the Proto-Tethys Ocean beneath the East Kunlun Orogen. Studying the I-type granites and mafic–ultramafic rocks formed during this period can provide valuable insights into the early Paleozoic tectonic evolution of the region. This paper incorporates petrology, LA-ICP-MS zircon U-Pb geochronology, and whole-rock major and trace element data obtained from the Kekesha intrusion in the eastern section of the East Kunlun Orogen. The formation age, petrogenesis, and magmatic source region of the intrusion are revealed, and the early tectonic evolution process of the subduction of the Proto-Tethys Ocean is discussed. The Kekesha intrusion includes four main rock types: gabbro, gabbro diorite, quartz diorite, and granodiorite. The zircon U-Pb ages are 515.7 ± 7.4 Ma for gabbro, 508.9 ± 9.8 Ma for gabbro diorite, 499.6 ± 4.0 Ma for quartz diorite, and 502.3 ± 9.3 Ma and 501.6 ± 6.2 Ma for granodiorite, respectively, indicating that they were formed in the Middle Cambrian. The geochemical results indicate that the gabbro belongs to the high-Al calc-alkaline basalt series, the gabbro diorite belongs to the medium-high-K calc-alkaline basalt series, the quartz diorite belongs to the quasi-aluminous medium-high-K calc-alkaline I-type granite series, and the granodiorite belongs to the weakly peraluminous calc-alkaline I-type granite series, all of which belong to the medium-high-K calc-alkaline series that have undergone varying degrees of differentiation and contamination. Gabbro and gabbro diorite exhibit significant enrichment in light rare earth elements (LREEs), depletion in heavy rare earth elements (HREEs), and an enhanced negative anomaly in Eu (Europium). Compared to gabbro and gabbro diorite, quartz diorite and granodiorite exhibit more pronounced enrichment in LREEs, more significant depletion in HREEs, and an enhanced negative anomaly in Eu. All four rock types are enriched in large-ion lithophile elements (LILEs) such as Cs, Rb, Th, Ba, and U, and are depleted in high-field-strength elements (HFSEs) such as Nb, Ta, and Ti. This indicates that these rocks originated from the same or similar mixed mantle source regions, and that they are formed in the island-arc tectonic environment. This paper suggests that the gabbro and gabbro diorite are mainly derived from the basic magma formed by partial melting of the lithospheric mantle metasomatized by subducted slab melt in the oceanic crust subduction zone and mixed with a small amount of asthenosphere mantle material. Quartz diorite results from the crystal fractionation of basic magma and experiences crustal contamination during magmatic evolution. Granodiorite forms through the crystal fractionation of basic magma, mixed with partial melting products from quartz diorite. While the lithology of the intrusions differs, their geochemical characteristics suggest they share the same tectonic environment. Together, they record the geological processes associated with island-arc formation in the East Kunlun region, driven by the northward subduction of the Proto-Tethys Ocean during the Early Paleozoic. Based on regional tectonic evolution, it is proposed that the Proto-Tethys Ocean began subducting northward beneath the East Kunlun block from the Middle Cambrian. The Kekesha intrusion formed between 516 and 500 Ma, marking the early stages of Proto-Tethys Ocean crust subduction.

Early to Middle Triassic sedimentary records in the Youjiang Basin, South China: Implications for Indosinian orogenesis

Abstract. A nonconformity refers to a hiatal surface located between metamorphic or igneous rocks and overlying sedimentary or volcanic rocks. These surfaces are key features with respect to understanding the relations among climate, lithosphere and tectonic movements during ancient times. In this study, the petrological, mineralogical and geochemical characteristics of Variscan basement rock as well as its overlying Permian volcano-sedimentary succession from a drill core in the Sprendlinger Horst, Germany, are analyzed by means of polarization microscopy, and environmental scanning electron microscope, X-Ray diffraction, X-ray fluorescence and inductively coupled plasma mass spectrometry analyses. In the gabbroic diorite of the basement, the intensity of micro- and macro-fractures increases towards the top, indicating an intense physical weathering. The overlying Permian volcanic rock is a basaltic andesite that shows less intense physical weathering compared with the gabbroic diorite. In both segments, secondary minerals are dominated by illite and a mixed-layer phase of illite and smectite (I–S). The corrected chemical index of alteration (CIA) and the plagioclase index of alteration (PIA) indicate an intermediate to unweathered degree in the gabbroic diorite and an extreme to unweathered degree in the basaltic andesite. The τ values for both basaltic andesite and gabbroic diorite indicate an abnormal enrichment of K, Rb and Cs that cannot be observed in the overlying Permian sedimentary rocks. Accompanying minerals such as adularia suggest subsequent overprint by (K-rich) fluids during burial diagenesis which promoted the conversion from smectite to illite. The overall order of element depletion in both basaltic andesite and gabbroic diorite during the weathering process is as follows: large-ion lithophile elements (LILEs) > rare earth elements (REEs) > high-field-strength elements (HFSEs). Concerning the REEs, heavy rare earth elements (HREEs) are less depleted than light rare earth elements (LREEs). Our study shows that features of supergene physical and chemical paleo-weathering are well conserved at the post-Variscan nonconformity despite hypogene alteration. Both can be distinguished by characteristic minerals and geochemical indices. Based on these results, a new workflow to eliminate distractions for paleoclimate evaluation and evolution is developed.

Petrogenesis of the Early-Middle Triassic high-Mg andesitic rocks in the southern margin of the South China Block: Implications for the convergence between the South China and Indochina Blocks

ABSTRACTLate Triassic–Early Jurassic intrusions of the Erguna Block, Northeast China, are located along the southern margin of the Mongol–Okhotsk orogenic belt. They comprise granodiorite, monzogranite, syenogranite, and lesser gabbro–diorite, of adakitic and calc­alkaline affinity. The adakite-like and calc­alkaline granites share similar light rare earth elements (LREE) characteristics; however, their heavy rare earth elements (HREE) trends differ from one another. The relative abundances of HREE in the calc­alkaline granites are relatively consistent and are similar to those of intrusive rocks formed from dehydration melting of garnet-free amphibolitic source rocks at relatively low pressures. In contrast, the adakite-like granites show more prominent HREE fractionation trends, indicating that they crystallized at higher pressures, where garnet in the source rocks was stable. At least two isotopically distinct sources were involved in the petrogenesis of the granites, but the extent to which they contributed varies between plutons. Most intrusions have incorporated an isotopically primitive component, possibly juvenile mafic crust. The other sources include a small proportion of old continental crustal material and isotopically evolved wall rocks. The gabbro–diorites have high MgO contents (>7 wt.%), a high Mg# (>0.6), and show moderate LREE and HREE fractionation, indicating they formed from the melting of subducted metasomatized lithospheric mantle. All of the intrusions in the study area are characterized by a relative enrichment in large ion lithophile elements (LILE) and depletion in high field strength elements (HFSE), indicating they were emplaced in an Andean-type active continental margin setting related to southward subduction of the Mongol–Okhotsk oceanic plate.

The subduction initiation and the early‐stage tectonics of the southern Paleo‐Asian Ocean have rarely been discussed. To address these problems, we report new geochemical and geochronological data of arc‐related volcanic rocks in the Huaniushan arc of the southern Beishan. Basaltic andesites and high‐Mg rocks yield zircon U‐Pb ages of 449–422 Ma and 460–437 Ma, respectively. The basaltic andesite is a continental arc‐related tholeiite with moderate amounts of TiO2; and is enriched in light rare earth elements (LREE), such as Rb, Ba, Th and Sr; it is also depleted in Nb, Ta and Eu; and has moderate εHf (t) (−0.3 to +5.5) and εNd (t) (−3.48 to +0.03) values; and high (87Sr/86Sr)i (0.7066–0.7079) values. The high‐Mg andesite and dacite are sanukitic‐type high‐Mg rocks, which have high MgO (Mg# = 49–62) contents, A/CNK (0.82–2.36) and (La/Sm)N values, low Sr/Y (3–15) and Ba/Th (11–182) ratios, moderate to low εHf (t) (−16.7 to +7.6) and εNd (t) (−3.74 to +0.05), and high (87Sr/86Sr)i values. All geochemical data indicate that they were sourced from mixing between mantle‐peridotite‐derived melts and silicic melts generated by the subducting oceanic slab and/or sediments in a hot subduction setting. After integrating the Sanukitic high‐Mg andesites (HMAs) with previously reported Nb‐enriched basalt, volumes of arc‐related tholeiitic to calc‐alkaline volcanic rocks, I‐ to S‐type granites, and HT eclogites, we conclude that the subduction initiation was one of the most likely geodynamic process for the metamorphism and magmatism in the Middle Ordovician to early Silurian.

Geochronology and geochemistry of Neoproterozoic magmatism in the Erguna Massif, NE China: Petrogenesis and implications for the breakup of the Rodinia supercontinent