Post-collisional Magmatic Rocks Research Articles

Subduction polarity reversal facilitated by plate coupling during arc-continent collision: Evidence from the Western Kunlun orogenic belt, northwest Tibetan Plateau

Abstract Subduction polarity reversal usually involves the break off or tearing of the downgoing plate (DP) along the continent-ocean transition zone, in order to initiate subduction of the overriding plate (OP) with opposite polarity. We propose that subduction polarity reversal can also be caused by DP-OP coupling and can account for the early Paleozoic geological relationships in the Western Kunlun orogenic belt in the northwestern Tibetan Plateau. Our synthesis of elemental and isotopic data reveals transient (~2 m.y.) changes in the sources of early Paleozoic arc magmatism in the southern Kunlun terrane. The early-stage (ca. 530–487 Ma) magmatic rocks display relatively high εNd(t) (+0.3 to +8.7), εHf(t) (−3.6 to +16.0), and intra-oceanic arc-like features. In contrast, the late-stage (485–430 Ma) magmatic rocks have predominantly negative εNd(t) (−4.5 to +0.3), εHf(t) (−8.8 to +0.9), and higher incompatible trace elements (e.g., Th), similar to the sub-continental lithospheric mantle beneath the Tarim craton. This abrupt temporal-spatial variation of arc magmatism, together with the detrital zircon evidence, indicate that subduction polarity reversal of the Proto-Tethys Ocean occurred in a period of ~10 m.y., consistent with the time interval reflected by ophiolite age. This rapid polarity reversal corresponds with the absence of ultrahigh-pressure (UHP) metamorphic and post-collisional magmatic rocks, features normally characteristic of slab break-off or tearing. Numerical modeling shows that this polarity reversal was caused by plate coupling during arc-continent collision. This coupling modified the normal succession of arc-continent collision events, preventing slab break-off or tearing-induced buoyant rock rebound and asthenosphere upwelling. Our model successfully explains early Paleozoic orogenesis in the Western Kunlun orogenic belt and may be applied elsewhere where post-collisional magmatic and UHP rocks are absent.

Mg-Ca-Fe isotopes of post-collisional magmatic rocks record the crust-mantle interaction processes beneath southern Tibet

The non-traditional Mg, Ca and Fe isotopes have proven to be useful tools for investigating the petrogenesis of the mantle- and crust-derived rocks. Individual non-traditional isotopes in continental basalts are well-studied, particularly in subduction settings. In contrast, there is a significant lack of comprehensive research on various non-traditional stable isotopes related to magmatic processes within continental collision zones, especially during post-collisional stage. To address this knowledge gap, this study presents stable Mg-Ca-Fe isotopic data for the Miocene post-collisional magmatic rocks including ultrapotassic, potassic, and adakitic rocks from the Lhasa Terrane in southern Tibet. The ultrapotassic rocks exhibit slightly lower δ26Mg (−0.28 ± 0.07‰, 2SD, n = 18) and significantly lower δ44/40Ca (0.69 ± 0.03‰, 2SD, n = 18) values than the depleted mantle. The high CaO/Al2O3 and CaO/TiO2 ratios of the ultrapotassic rocks are typical characteristics of carbonate metasomatism, suggesting that the light Mg and Ca isotopic composition may be attributed to the involvement of recycled Ca-rich sedimentary carbonate in the mantle source. Additionally, these ultrapotassic rocks display δ56Fe values (0.026–0.163‰, 2SD, n = 18) ranging from the normal peridotite mantle to much higher values, suggesting a likely origin from a pyroxenite-bearing lithospheric mantle source. These findings imply that subducted oceanic crust and overlying Ca-rich sediments plausibly account for the formation of a mantle source characterized by distinctive Mg-Ca-Fe isotope compositions. The adakitic rocks exhibit the highest δ26Mg (−0.13 ± 0.10‰, 2SD, n = 15) and δ56Fe (up to 0.304‰) values, along with the lowest δ44/40Ca values (0.60 ± 0.07‰, 2SE, n = 15) among these post-collisional magmatic rocks. They also demonstrate strong correlations between δ56Fe and (Gd/Lu)N ratios, indicating a significant role of garnet in their source, consistent with their origin from the thickened lower crust associated with the collision between India and Asia continents. Given the consistent δ56Fe values (0.102 ± 0.067‰, 2SD, n = 5) observed in the adakitic rocks from the western Lhasa Terrane, we posit that it reflects the Fe isotopic composition of the thickened lower crust in southern Tibet. The potassic rocks display lighter Ca isotopes (δ44/40Ca = 0.67 ± 0.03‰, 2SE, n = 5) than the mantle, consistent with a carbonate metasomatized source. They exhibit varying δ26Mg (−0.31 ± 0.07‰ to −0.04 ± 0.07‰, 2SD) and relatively uniform δ56Fe (0.186 ± 0.051‰, 2SE, n = 6) values, falling within the range of nearly contemporaneous ultrapotassic and adakitic rocks. This suggests that potassic rocks may be formed through the mixing of mantle materials resembling ultrapotassic rocks and thickened crustal components resembling adakitic rocks in the lower crust. Building upon these findings, we propose that the distinctive geochemical characteristics, as well as the spatial and temporal distribution of post-collisional magmatic rocks in southern Tibet, are the outcome of partial melting within a carbonated pyroxenite-bearing lithospheric mantle, coupled with the involvement of thickened crust and their subsequent interactions.

Age, clinopyroxene geochemistry and petrogenesis of post-collisional magmatic rocks in the Jonob-e-Sechangi area, Lut block, eastern Iran

The Lut-Sistan region, eastern Iran has an extensive record of Eocene-Oligocene magmatism the origin of which is not fully understood. In this study, we analyze a set of volcanic rocks in the Jonob-e-Sechangi area of the Lut block. U-Pb zircon analyses demonstrate that magmatism can be divided into two age groups. Group 1 formed at ∼45 Ma and involved different felsic (calc-alkaline rhyolitic to dacitic) and intermediate (high-K calc-alkaline andesitic) rocks. All samples from Group 1 display nearly identical initial 87Sr/86Sr values (87Sr/86Sri = 0.7045-0.7056). However, the felsic rocks show higher ԐNdt (+2.21 to +2.73) and lower TDM (0.51-0.61 Ga). In contrast, intermediate rocks have ԐNdt values of +1.27 to +2.23 and TDM values between 0.63 and 0.74 Ga. Additional geochemical variations suggest that the felsic and intermediate rocks of Group 1 are not co-genetic. The felsic extrusive rocks in the Jonob-e-Sechangi area likely formed from the partial melting of the amphibolitic arc crust. In contrast, the high-K calc-alkaline andesitic rocks of Group 1 may have originated from low-degree partial melting of a spinel-peridotite mantle source.Group 2 formed basaltic andesite to andesitic rocks at37-40 Ma, showing high-K calc-alkaline to shoshonitic compositions, relatively high 87Sr/86Sri (0.7047- 0.7076) and relatively low ԐNdt (0.48 to-5.86). These geochemical features require magma derivation from a mantle source that is not only elementally but also isotopically enriched, followed by variable fractionation and crustal contamination. The enriched mantle source was most likely resided in the lithosphere and formed by metasomatism by continental crust-derived materials. Clinopyroxene trace elements compositions display marked variations between different magmatic rocks. Clinopyroxene in dacitic samples of Group 1 shows relatively high contents of Sr, Rb, and Ba whereas that in Group 2 rocks has relatively high contents of Sc, V, Ga, Nb, Zr, Hf, Y and rare earth elements (REE). The findings of this study show that the intricate interactions between the lithosphere and asthenosphere, stemming from post-collisional delamination processes following the closure of the Sistan Ocean and the collision of the Lut-Afghan blocks, led to differing levels of partial melting in the region's heterogeneous metasomatized mantle. This phenomenon explains the diverse range of volcanic rocks found not just in the Jonob-e-Sechangi area but also in other regions of the Lut Block.

Petrogenesis of Early Cretaceous andesites and mafic dikes in central Tibet: Implications for the growth of continental crust in collision zones

Post-collisional magmatic rocks in central Tibet provide an opportunity to study the processes of continental crustal growth in a collision zone. Here, we report data for samples of andesites and coeval mafic dikes obtained from the Jiaqiong area of the Bangong–Nujiang suture zone, central Tibet. Zircon U–Pb dating shows that the andesites and mafic dikes were emplaced concurrently during the late Early Cretaceous (ca. 115 Ma). The andesites have high Mg# values (55.2–61.6) and initial 87Sr/86Sr ratios (0.7086–0.7088), negative whole-rock εNd(t) (−6.5 to − 6.1) and zircon εHf(t) (−5.1 to − 1.7) values, and elevated zircon δ18O (6.8‰–7.5‰) values, indicating that the andesites were probably derived by the partial melting of an ancient subduction-modified lithospheric mantle. The coeval mafic dikes have moderately high Mg# values (49.3–50.0) and initial 87Sr/86Sr ratios (0.7076–0.7082), and negative εNd(t) values (−5.4 to − 5.3), implying that they were derived from an enriched lithospheric mantle source. However, the zircon εHf(t) (−5.8 to + 1.6) and δ18O (5.2‰–6.3‰) values of these mafic dikes indicate the influence of depleted mantle components. Furthermore, results of Sr–Nd–Hf–O isotopic mixing modeling suggest that these mafic dikes could have been produced by interaction between asthenospheric and lithospheric mantle. Therefore, we suggest that the parental magmas of the mafic dikes were probably generated by interaction between asthenospheric and overlying metasomatized lithospheric mantle. Combined with data on nearby Cretaceous magmatic rocks and sedimentary strata, we propose that the studied magmatic rocks were formed in a post-collisional slab breakoff setting in central Tibet and have made a substantial contribution to the vertical growth of continental crust in this continental collision zone.

Lithosphere tearing and foundering during continental subduction: Insights from Oligocene−Miocene magmatism in southern Tibet

The distribution of Oligo−Miocene magmatic rocks from southern Tibet in space and time yields critical information on the geometry and deformation of the subducted Indian lithosphere which impacts on plateau growth following the India and Eurasia collision. A growing body of geophysical evidence has shown that the subducted Indian lithosphere beneath the Tibetan Plateau has been torn apart. However, the spatiotemporal distribution and cause of the tearing remain enigmatic. Timing of the post-collisional magmatic rocks in southern Tibet exhibits four patterns of decreasing ages; magmatism began earlier in the west and east Himalayan syntaxis and evolved to two age undulations in the central southern Tibet. Seismic images show that regions of slab window (both 90°E and 84°E) and flattened subducted lithosphere (both 86°E and 81°E) are present at depth of 135 km. Correspondingly, increasing mineral crystallization temperatures (absolute value of 50 °C) were recorded in the Oligo−Miocene ultrapotassic-potassic rocks at 90°E and 84°E, while opposing trends were shown by coeval ultrapotassic-potassic rocks at 86°E and 81°E. Besides, the melting depth of the Oligo−Miocene ultrapotassic-potassic primitive melts decreases from nearly 100 km to 70 km between 81°E and 90°E, probably indicating progressive rising of the lithosphere-asthenosphere boundary. Such variations were possibly the results of the focused flow and upwelling of asthenosphere, which advanced rapidly but diachronously through weakened and torn sectors within the overlying Indian slab. The upwellings probably induced diachronously upward bending of the residual Indian slab and its flattening, which accelerated the tearing of the Indian lithosphere during continental subduction.

Tectono-Magmatic Significance of the Lower Devonian Mafic Intrusions in the East Kunlun Orogenic Belt: Keys for the Evolution of Proto-Tethys

Studies on post-collisional magmatic rocks can provide key clues to researching the crust–mantle interactions and the tectonic evolution of collisional orogenic belts. This study investigated a suite of newly discovered mafic intrusions in the middle of the East Kunlun orogenic belt through integrated analysis of petrology, petrography, and zircon U–Pb dating. The data could offer new insights into the generation of the Proto-Tethyan tectonic evolution. The result shows that these mafic intrusions are mainly gabbro and diabase, formed in the Early Devonian, with zircon U–Pb ages of 408.9 ± 2.0 Ma for gabbro and 411.1 ± 3.1 Ma for diabase. It consists of plagioclase, pyroxene, and dark minerals, and a small number of calcite and chlorite. Diabase has a small amount of amygdale. Their Na2O + K2O contents range from 3.47 wt.% to 5.45 wt.%, with Na2O/K2O ratios from 1.39 to 3.09, suggesting that they are calc–alkaline rocks. These rocks have an Fe2O3ᵀ content of 7.68 wt.%–11.59 wt.% and Mg# of 50.58–59.48, belonging to the iron-rich and magnesium-poor type. The chondrite-normalized rare earth elements show similar patterns that are characterized by enrichment of light rare earth elements, with (La/Yb)N of 3.27–6.75 and no significant europium anomaly, indicating the rocks are homogenous. The studied rocks are characterized by low contents of compatible elements Cr and Ni, enrichment of large-ion lithophile elements such as Rb, U, Sr, and Nd, and high-field-strength elements such as Nb, Ta, Zr, Hf, and Th. The mafic magma originated from the partial melting of the enriched mantle and was assimilated and mixed with crust materials during the process of migration. Based on the regional tectonic evolution, we interpret that the Proto-Tethys Ocean had closed in the Early Devonian, and that the East Kunlun region was in a post-collisional extensional tectonic setting.

U-Pb geochronology of a reversely zoned pluton: Records of pre-to-post collisional magmatism of the Araçuaí belt (SE-Brazil)?

Post-collisional reversely zoned plutons occurring in the Araçuaí belt frequently show basic cores enveloped by felsic rocks, with intense magma mingling/mixing features. The Venda Nova (VN) pluton presents, in addition to its mafic core composed of gabbronorites surrounded by syenomonzonites, a narrow ring of charnockite and norite. The VN is considered as a marker of the last magmatic phases of the Araçuaí orogenic collapse and bears one of the few high-quality Cambrian paleomagnetic data for the West Gondwana, but no geochronological data is yet available for it. In this paper, we report new in-situ U-Pb geochronological data (SHRIMP II) for the syenomonzonite, gabbro and charnockite of the Venda Nova pluton. These data are discussed and compared to the geochemical and geochronological data available for the post-collisional magmatic rocks of the Araçuaí orogen. The concordant ages of the syenomonzonite (496.9 ± 4.2 Ma, MSWD = 0.0019, n = 14) and gabbro (500.6 ± 3.7 Ma, MSWD = 0.0290, n = 14) are likely the crystallization age of the main plutonic body and corroborate the previous interpretation of paleomagnetic data. In contrast, two age groups were identified for the charnockite: Group I (n = 14, 624.0 ± 3.1 Ma, MSWD = 0.116), which includes zircon borders and rims without evidence of border reaction, and Group II (n = 6, 483.7 ± 7.4 Ma, MSWD = 0.0003) from zircon grains with luminous rims and older cores. Since the Venda Nova charnockite has (i) a distinct geochemical signature compared to other post-collisional charnockite bodies found in the same region, (ii) higher magnetic anisotropy degrees compared to other border units of the same intrusion, and (iii) marked sub-solidus deformation features, the Group I ages are suggested as the crystallization age of these rocks, while Group II ages would be better interpreted as a resetting due to the thermal influence of the syenomonzonite and gabbro intrusions. This implies that the VN is a composite magmatic complex with contributions of magmas from the pre-collisional magmatic arc and from the post-collisional melts originated in the crust and mantle across the Araçuaí lithosphere during orogen collapse.

Carboniferous to Early Permian magmatism in the Uliastai continental margin (Inner Mongolia) and its correlation with the tectonic evolution of the Hegenshan Ocean

This study presents a comprehensive analysis of zircon U–Pb–Hf isotopic and whole-rock geochemical data of Carboniferous–Early Permian felsic igneous rocks from the Uliastai continental margin (UCM), southeastern Central Asian Orogenic Belt, to constrain the amalgamation of the Inner Mongolia–Daxing'an Orogenic Belt (IMDOB) in the late Paleozoic. Zircon laser ablation inductively coupled plasma mass spectrometry UPb ages of monzogranite, granite porphyry, felsic volcanic rock, and alkali feldspar granite reveal three stages of magmatism in the Early Carboniferous (ca. 336–320 Ma), Late Carboniferous (ca. 311–307 Ma), and Early Permian (ca. 298 Ma). The Early Carboniferous igneous rocks, including monzogranites and later granite porphyries, exhibit affinity with the high-K calc-alkaline highly fractionated I-type granite, and are depleted in high-field-strength elements and enriched in large-ion lithophile elements. Their zircon εHf(t) values of the Early Carboniferous rocks range from +4.6 to +9.1, indicating that they were generated by partial melting of the juvenile crustal materials in a north-dipping subduction-related environment along with the opening of the Hegenshan Ocean. The Late Carboniferous felsic volcanic rocks are highly fractionated high-K calc-alkaline to alkali-calcic I- and A-type granitoids, with relatively strong peraluminous affinities. Considering the zircon εHf(t) values (+6.2 to +10.6) and relatively low Rb/Ba (0.71–3.57) and Rb/Sr (4.18–8.12) ratios, they were likely derived from a juvenile crust and are comparable to partial melts of clay-poor but plagioclase-rich metaigneous rocks. Notably, the A-type granitoids are gradually increasing in the UCM after 311 Ma, and these magmatic rocks have markedly increased zircon saturation temperatures and (K2O + Na2O)/CaO ratios. These changes may result from the slab break-off of the Hegenshan Ocean, and the Late Carboniferous felsic volcanic rocks were formed in a post-collisional setting. The Early Permian alkali feldspar granites are characterized by typical A-type granite geochemistry, with high Ga/Al ratios; low MgO, Cr, Co, and Ni contents; and high zircon saturation temperatures. These rocks have positive εHf(t) values of +10.2 to +12.7 and record an intracontinental extension setting. In combination with the regional geology, the Late Carboniferous post-collisional magmatic rocks suggest that a change from a subduction-related continental margin to a post-collisional tectonic regime might have occurred in the UCM before 311 Ma.

Crustal architecture of the southern Tongbai orogen, central China: Insight from migmatites and post-collisional granites

The crustal structure of collisional orogen is critical for deciphering the crustal evolution and geodynamics of an orogen. Metamorphic rocks and post-collisional magmatic rocks are the key subjects to probe the crustal structure. In this study, we conducted an integrated study of zircon UPb age, Hf and O isotopes for migmatites and post-collisional granites in the southern segment of the Tongbai orogen. Zircon U−Pb dating yields protolith ages of 755 ± 55 to 713 ± 6 Ma and metamorphic ages of 162 ± 11 Ma to 138 ± 2 Ma for the migmatites and intrusion ages of 137 ± 2 Ma for the post-collisional granites. The inherited magmatic cores in the migmatites exhibit similar Hf isotopic compositions to the metamorphic zircons. Their εHf(t) values range from 13.5 to −9.5 with crust Hf model ages of 787 Ma to 2233 Ma, indicating that the protoliths were derived from reworking of Paleoproterozoic crust with the addition of juvenile components during the Neoproterozoic. Zircon δ18O of inherited magmatic cores are of −1.45‰ to 5.90‰, signifying the protoliths were formed by remelting of hydrothermally altered low δ18O rocks. By comparison, the post-collisional granites mostly show normal mantle-like O zircon isotope compositions and have lower εHf(t) and εNd(t) values than the migmatite rocks. These features suggest that the granites were formed by fusion of the ancient crust that might have suffered insufficient water-rock interaction. The discrepancies in O–Hf–Nd isotopic compositions of the studied migmatites and granites might be inherited from their precursors. It is thus inferred that the precursors of the granites might lie at the shoulder of the Neoproterozoic rift and thus might have experienced slight water-rock hydrothermal alteration and crust-mantle interaction; whereas the protoliths of the metamorphic rocks might be located at or near the rift center and thus had suffered intensive water-rock hydrothermal alteration and crust-mantle interaction. Prior to the late Mesozoic magmatism, the Tongbai orogen would have an uncoupled double-layered crust, where the upper layer consists of the metamorphic rocks and the lower is the sources of the post-collisional granites. They were stacked by the Permian-Triassic orogeny. The final architecture of the Tongbai orogen might be constituted by the emplacement of the voluminous granites from the deeper crust and the formation of the migmatites in the Tongbai complex. This configuration might widespread for other collisional orogens.

Late Eocene post-collisional magmatic rocks from the southern Qiangtang terrane record the melting of pre-collisional enriched lithospheric mantle

Abstract Diverse rock types and contrasting geochemical compositions of post-collisional mafic rocks across the Tibetan Plateau indicate that the underlying enriched lithospheric mantle is heterogeneous; however, how these enriched mantle sources were formed is still debated. The accreted terranes within the Tibetan Plateau experienced multiple stages of evolution. To track the geochemical characteristics of their associated lithospheric mantle through time, we can use mantle-derived magmas to constrain the mechanism of mantle enrichment. We report zircon U-Pb ages, major and trace element contents, and Sr-Nd isotopic compositions for Early Cretaceous and late Eocene mafic rocks in the southern Qiangtang terrane. The Early Cretaceous Baishagang basalts (107.3 Ma) are characterized by low K2O/Na2O (<1.0) ratios, arc-like trace element patterns, and uniform Sr-Nd isotopic compositions [(87Sr/86Sr)i = 0.7067–0.7073, εNd(t) = −0.4 to −0.2]. We suggest that the Baishagang basalts were derived from partial melting of enriched lithospheric mantle that was metasomatized by subducted Bangong–Nujiang oceanic material. We establish the geochemistry of the pre-collisional enriched lithospheric mantle under the southern Qiangtang terrane by combining our data with those from other Early Cretaceous mafic rocks in the region. The late Eocene (ca. 35 Ma) post-collisional rocks in the southern Qiangtang terrane have low K2O/Na2O (<1.0) ratios, and their major element, trace element, and Sr-Nd isotopic compositions [(87Sr/86Sr)i = 0.7042–0.7072, εNd(t) = −4.5 to +1.5] are similar to those of the Early Cretaceous mafic rocks. Based on the distribution, melting depths, and whole-rock geochemical compositions of the Early Cretaceous and late Eocene mafic rocks, we argue that the primitive late Eocene post-collisional rocks were derived from pre-collisional enriched lithospheric mantle, and the evolved samples were produced by assimilation and fractional crystallization of primary basaltic magma. Asthenosphere upwelling in response to the removal of lithospheric mantle induced the partial melting of enriched lithospheric mantle at ca. 35 Ma.

Miocene Olivine Leucitites in the Hoh Xil Basin, Northern Tibet: Implications for Intracontinental Lithosphere Melting and Surface Uplift of the Tibetan Plateau

Abstract The generation of Miocene–Pliocene post-collisional magmatic rocks in northern Tibet was coeval with surface uplift, meaning that understanding the petrogenesis of these rocks should provide clues to the mechanism of uplift of the Tibetan Plateau. However, the nature of the source(s) of Miocene–Pliocene post-collisional rocks is unresolved, especially for potassic–ultrapotassic rocks. This study focuses on 16 Ma olivine leucitites in the Hoh Xil Basin of northern Tibet, which display the lowest SiO2 (43·4–48·8 wt%) contents of all Miocene–Pliocene magmatic rocks in northern Tibet and have high MgO (4·85–8·57 wt%) contents and high K2O/Na2O (>1) ratios. Whole-rock geochemical compositions suggest that the olivine leucitites did not undergo significant fractional crystallization or crustal assimilation. All samples are enriched in large ion lithophile elements relative to high field strength elements, and they exhibit uniform whole-rock Sr–Nd isotope [(87Sr/86Sr)i = 0·7071–0·7077 and εNd(t) = −3·1 to −3·9] and olivine O isotope (5·8–6·6 ‰, mean of 6·2 ± 0·2 ‰, n = 21) compositions. We propose that the olivine leucitites were derived by low-degree partial melting of phlogopite-lherzolite in garnet-facies lithospheric mantle. Given the tectonic evolution of the Hoh Xil Basin and adjacent areas, we suggest that southward subduction of Asian (Qaidam block) lithosphere after India–Asia collision transferred potassium and other incompatible elements into the lithospheric mantle, forming the K-enriched mantle source of the Miocene–Pliocene potassic–ultrapotassic rocks. Removal of lower lithospheric mantle subsequently induced voluminous Miocene–Pliocene magmatism and generated >1 km surface uplift in the Hoh Xil Basin.

Late Oligocene–early Miocene transformation of postcollisional magmatism in Tibet

Abstract Uplift of the Tibetan Plateau is thought to be one of the most important orogenic and climate forcing events of the Cenozoic Era, associated with geodynamic changes related to India-Asia collision and subsequent continental lithosphere subduction. However, the fate and scale of the subducted continental lithosphere segments remain highly controversial. Using a comprehensive compilation of the spatiotemporal distribution of postcollisional magmatic rocks across Tibet, together with new geochemical and Sr-Nd-Pb isotopic data and modeling simulations, we propose a holistic, two-stage evolutionary model to explain the link between genesis of the magmas and continental subduction. The magmatism prior to 25 Ma resulted from continuous upwelling of a carbonate-rich upper-mantle plume induced by northward underthrusting of Indian oceanic and continental lithosphere with its cover of Tethyan platform carbonate sediments, whereas magmatism after 25 Ma was related to opposing north-directed and south-directed continental subduction. Our model indicates a transformation in the distribution and nature of the magmatism in Tibet at ca. 25 Ma, which reflects a significant change in the Himalayan-Tibetan orogen and associated mantle dynamic processes in the early Miocene. Understanding this transformation could have important implications for the utility of the Himalayan-Tibetan system as a modern analogue for ancient orogens.

Provenance and tectonic setting transition as recorded in the Neoproterozoic strata, western Jiangnan Orogen: Implications for South China within Rodinia

The Neoproterozoic Tonian strata (ca. 870–725 Ma) in the western Jiangnan Orogen archive the records of sedimentary provenance and tectonic setting which can be used to understand the geological evolution of the South China Continent. These strata are separated into the basement and cover sequences by a regional angular unconformity. The basement sequence can be subdivided into the lower and the upper parts by the widespread interbedded ca. 840 Ma basalt with pillow structure. In the present work, 234 concordant detrital zircon analyses are obtained from three Tonian sandstone samples in the Fanjingshan district, Guizhou Province. Combined with previous results, a total of 1736 analyses of detrital zircon U-Pb ages derived from 12 formations of Tonian strata in the western Jiangnan Orogen are used to decipher the integrated sedimentary and tectonic histories. The zircons from the lowermost part of the basement sequence (the Yujiagou Formation) show oval morphology and display two Paleoproterozoic age peaks at 2325 Ma and 1845 Ma which are similar with the detrital zircon age peaks from the Late Paleoproterozoic to Early Mesoproterozoic Dongchuan/Dahongshan/Hekou groups, suggesting a passive margin basin in which the sediments were mainly sourced from the southwestern Yangtze Block. However, the zircon age population of the lower part of the basement sequence (the Xiaojiahe, Huixiangping formations and their equivalents) indicates the sedimentary derivation from bidirectional sources (the ca. 870 Ma arc materials in the south and the old detritus from the southwestern Yangtze Block) which is consistent with a back arc setting for the deposition of the sediments. Zircons from the upper part of the basement sequence (the Duyantang Formation and its equivalent) show euhedral and subangular morphology and display a unimodal age peak at ca. 835 Ma. This sequence was possibly deposited in a convergent setting and the detritus were came from the locally distributed syn-collisional igneous rocks. The lower part of the cover sequence (the Xinzhai and Wuye formations and their equivalents) shows a distinct zircon age peak at 815–809 Ma and two subordinate peaks at 2485 Ma and 2018 Ma, suggesting that the basin had gradually transformed into a continental rift basin and received the detritus from the ca. 815 Ma post-collisional magmatic rocks as well as from different Paleoproterozoic source rocks in the northern Yangtze Block. We propose a tectonic evolution model that envisages eruption of ca. 840 Ma basalt in a back arc basin that existed during ca. 870–835 Ma, an angular unconformity was formed during amalgamation of the Yangtze Block and the Cathaysia Block at ca. 835–820 Ma and the rifting of the South China Continent was initiated at ca. 800 Ma. Our study concludes that the South China Continent was formed on the periphery of the Rodinia supercontinent.

Detrital zircon age spectra of the Gurvan Sayhan accretionary complex in South Mongolia: Constraints on the Late Paleozoic evolution of the southern Central Asian Orogenic Belt

The southern Central Asian Orogenic Belt (CAOB) in South Mongolia consists of continental fragments sutured together by Paleozoic arc magmatism generated by the closing of the Paleo-Asian Ocean (PAO). Controversy persists regarding the timing and position of the final amalgamation of the Altai and Tianshan orogenic belts. This paper presents zircon U-Pb data of the Gurvan Sayhan accretionary complex, as well as the overlying sedimentary sequence, and a granite pluton intruding the complex to provide new insights into the Late Paleozoic evolution of South Mongolia. The detrital zircon grains from the Gurvan Sayhan accretionary complex yielded age spectra with predominant peaks at ca. 331 Ma, subdominant peaks at ca. 381, and 500 Ma, indistinct Precambrian peaks at ca. 848 Ma and the youngest group of 318 Ma. The Paleozoic zircon grains are interpreted to be derived from adjacent arc systems formed by the subduction of the PAO, whereas those with the Precambrian ages were possibly from the nearby Hangay and South Gobi microcontinent blocks. The proportion of Precambrian detrital zircon grains in the complex decreases with time, likely suggesting a change in the paleogeography during sedimentation of the complex. Detrital zircon grains from the overlying stratum produced an age spectrum characterized by containing the youngest peak at ca. 293 Ma, although older peaks at ca. 340 Ma and 466 Ma are also present. The granite pluton intruding the accretionary complex has an emplacement age of 292.9 ± 1.3 Ma, similar with the youngest detrital zircon age peak from the overlying stratum, which is interpreted from the post-collisional magmatic rocks. Therefore, our results demonstrate that the final closure of the PAO in southern Mongolia was Late Carboniferous–Early Permian (318–293 Ma), and the Gurvan Sayhan accretionary complex represents the suture zone of the Altai and Tianshan orogenic belts.

A geotraverse across two paleo-subduction zones in Tien Shan, Tajikistan

We present first LA-ICP-MS U–Pb zircon ages as well as geochemical and Sr–Nd–Pb isotope data for 14 magmatic rocks collected along ca. 400km profile across the Chatkal-Kurama terrane in the Mogol-Tau and Kurama ranges and the Gissar Segment of the Tien Shan orogen in Tajikistan. These new data from supra-subduction and post-collisional magmatic rocks of two Late Paleozoic active margins constrain a tectonic model for terrane motions across two paleo-subduction zones: (1) The 425Ma old Muzbulak granite of the Mogol-Tau range formed in a supra-subduction setting at the northern margin of the Turkestan Ocean. The north-dipping plate was subducted from the Early Silurian to the earliest Middle Devonian. Thereafter the northern side of the Turkestan Ocean remained a passive margin until the Early Carboniferous. (2) In the Early Carboniferous, subduction under the northern margin of the Turkestan Ocean resumed and the 315 to 305Ma old Kara-Kiya, Muzbek, and Karamazar intrusions formed in a supra-subduction setting in the Mogol-Tau and Kurama ranges. (3) At the same time, in the Early Carboniferous, rifting of the southern passive margin of the Turkestan Ocean formed the short-lived Gissar Basin, separated from the Turkestan Ocean by the Gissar micro-continent. North-dipping subduction in the Gissar Basin is documented by the 315Ma Kharangon plagiogranite and the voluminous ca. 321–312Ma Andean-type supra-subduction Gissar batholith. The Kharangon and Khanaka gabbro-plagiogranite intrusions of the southern Gissar range have geochemical and Sr–Nd isotopic compositions (87Sr/86Sr(t) 0.7047–0.7056, εNd of +1.5 to +2.3) compatible with mantle-derived origin typical for plagiogranites associated with ophiolites. The supra-subduction rocks from the Gissar batholith and from the Mogol-Tau Kurama ranges have variably mixed Sr–Nd–Pb isotopic signatures (87Sr/86Sr(t) 0.7057–0.7064, εNd of −2.1 to −5.0) typical for continental arcs where mantle-derived magmas interact with continental crust. (4) In the latest Carboniferous, the Turkestan Ocean and the Gissar Basin were closed. The Early Permian Chinorsay (288Ma) and Dara-i-pioz (267Ma) post-collisional intrusions, emplaced in the northern part of the Gissar micro-continent after a long period of amagmatic evolution, have intraplate geochemical affinities and isotopic Sr–Nd–Pb isotopic compositions (87Sr/86Sr(t) 0.7074–0.7086, εNd of −5.5 to −7.4) indicating derivation from Precambrian continental crust which is supported by old Nd model ages (1.5 and 1.7Ga), and by the presence of inherited zircon grains with ages 850–500Ma in the Chinorsay granodiorite. The post-collisional intrusions in the southern Gissar and in the Mogol-Tau and Kurama ranges (297–286Ma), emplaced directly after supra-subduction magmatic series, have geochemical and isotopic signatures of arc-related magmas. The distinct shoshonitic affinities of post-collisional intrusions in the Mogol-Tau and Kurama ranges are explained by the interaction of hot asthenospheric material with subduction-enriched wedge of lithospheric mantle due to slab break-off at post-collisional stage. Despite origination from different tectonic environments, all magmatic rocks have relatively old Nd model ages (1.7–1.0Ga) indicating a significant proportion of Paleoproterozoic or older crustal material in their sources and their model ages are similar to those of post-collisional intrusions from the Alai and Kokshaal Segments of the South Tien Shan.

Geochronology and geochemistry of late Carboniferous volcanic rocks from northern Inner Mongolia, North China: Petrogenesis and tectonic implications

Zircon U–Pb ages, geochemical and Sr–Nd isotopic data are presented for the late Carboniferous Baoligaomiao Formation (BG Fm.) and Delewula Formation (DW Fm.) volcanic rocks, widely distributed in northern Inner Mongolia, in the northern part of the Xing'an–Mongolia Orogenic Belt (XMOB). The BG Fm. rocks mainly consist of basaltic andesites and andesites while the DW Fm. rocks include dacites, trachytes, rhyolites, pyroclastic rocks and minor andesites. New LA-ICPMS zircon U–Pb analyses constrain their eruption to late Carboniferous (317–322Ma and 300–310Ma, respectively). The BG Fm. volcanic rocks are characterized by enriched large ion lithophile elements (LILE) and depleted high field strength elements (HFSE), with initial 87Sr/86Sr ratios of 0.70854–0.70869 and negative εNd(t) (−2.1 to −2.4) values. They have low La/Ba (0.03–0.05), high La/Nb (2.05–3.70) ratios and variable Ba/Th (59.5–211) ratios. Such features suggest that they are derived from melting of heterogeneous sources including a metasomatized mantle wedge and Precambrian crustal material. The DW Fm. volcanic rocks are more depleted in HFSE with significant Nb, Ta, P, Ti anomalies. They have high initial 87Sr/86Sr ratios (0.72037–0.72234) and strong negative εNd(t) (−11 to −11.6) values which indicate those igneous rocks were mainly derived from reworking of the Paleoproterozoic crust. The late Carboniferous volcanic rocks have geochemical characteristics similar to those of the continental arc rocks which indicate the northward subduction of the Paleo Asian Ocean may have continued to the late Carboniferous. The volcanic association of this study together with the early Permian post-collisional magmatic rocks suggests that a tectonic transition from subduction-related continental margin arc volcanism to post-collisional magmatism occurred in the northern XMOB between the late Carboniferous and the early Permian.

Developing plate tectonics theory from oceanic subduction zones to collisional orogens

Crustal subduction and continental collision is the core of plate tectonics theory. Understanding the formation and evolution of continental collision orogens is a key to develop the theory of plate tectonics. Different types of subduction zones have been categorized based on the nature of subducted crust. Two types of collisional orogens, i.e. arc-continent and continent-continent collisional orogens, have been recognized based on the nature of collisional blocks and the composition of derivative rocks. Arc-continent collisional orogens contain both ancient and juvenile crustal rocks, and reworking of those rocks at the post-collisional stage generates magmatic rocks with different geochemical compositions. If an orogen is built by collision between two relatively old continental blocks, post-collisional magmatic rocks are only derived from reworking of the old crustal rocks. Collisional orogens undergo reactivation and reworking at action of lithosphere extension, with inheritance not only in the tectonic regime but also in the geochemical compositions of reworked products (i.e., magmatic rocks). In order to unravel basic principles for the evolution of continental tectonics at the post-collisional stages, it is necessary to investigate the reworking of orogenic belts in the post-collisional regime, to recognize physicochemical differences in deep continental collision zones, and to understand petrogenetic links between the nature of subducted crust and post-collisional magmatic rocks. Afterwards we are in a position to build the systematics of continental tectonics and thus to develop the plate tectonics theory.

Postcollisional potassic and ultrapotassic rocks in southern Tibet: Mantle and crustal origins in response to India–Asia collision and convergence

Postcollisional potassium-rich magmatism in southern Tibet provides an important insight into the deep processes inducing accelerated plateau uplift and associated geologic episodes during early Miocene. As a major outcropping of postcollisional magmatic rocks in the southwestern Tibet, ultrapotassic and potassic volcanism in Xungba basin occurred coevally at ∼23Ma, consisting of bimodal-like volcanic sequence. The mantle-derived ultrapotassic rocks (Group 1) are latites and marked by both mantle and crustal geochemical signatures, similar to other younger (19–10Ma) ultrapotassic rocks elsewhere in the southern Tibet. The high Th/Yb ratios, low Ba/La and Hf/Sm ratios observed in ultrapotassic rocks strongly imply derivation from a metasomatized lithospheric mantle regions enriched by inputs of pelagic sediment and carbonate during previous Tethyan oceanic subduction, while their relatively high SiO2 and Dy/Yb, low Ni/MgO and CaO/Al2O3, and convex upward 87Sr/86Sr–δ18OV-SMOW relationship suggest additional contributions of overthickened lower crust and ancient basement except for enriched mantle sources. The potassic rocks (Group 2), which underlie Group 1 rocks, are intermediate to silicic and exhibit adakitic geochemical affinities with high Sr/Y and La/Yb ratios, and low Y and Yb concentrations. The fingerprint of overthickened lower crust identified both from ultrapotassic and potassic rocks may be an important feature for magmatism occurred under the background of continental collision. And the bimodal volcanic sequence of Xungba postcollisional magmatism may further corroborate that the removal of lower part of over-thickened lithospheric mantle at depth have triggered an extension setting associated with Miocene plateau uplift.

Miocene post-collisional shoshonites and their crustal xenoliths, Yarlung Zangbo Suture Zone southern Tibet: Geodynamic implications

The convergence between the Indian plate and the southern margin of the Eurasian continent created an active continental margin from Late Jurassic until about 40Ma ago, which then evolved to form the Himalaya and the Tibetan Plateau during the continental collision stage. Post-collisional magmatism in southern Tibet, north of the Yarlung Zangbo Suture Zone (YZSZ) has been active since 45Ma and is related to normal faulting and extensional tectonism. To date no such magmatism was reported within the YZSZ itself. This paper reports on the discovery of Miocene shoshonites within the YZSZ. They are significant because the magma traveled, at least in part, through oceanic crust, thus limiting interaction with the continental crust to the mid-crustal level and which affected the post-collisional magmatic rocks occurring in the northern part of the subduction system. In addition, xenoliths and xenocrysts of crustal origin in these rocks constrain the nature of metamorphic rocks underlying the YZSZ at mid-crustal level. The geochemical signatures of the shoshonitic rocks, including Nd and Sr isotope systematics, indicate derivation from a garnet-bearing middle continental crustal source. Crustal imprint complicates modeling of the petrogenetic processes which occurred prior to mid-crustal ponding of the magma which took place between 11 and 17Ma at depths of 40 to 50km. The significant role of crustal contamination raises serious concerns about models proposed for similar magmatic activity elsewhere in the Himalaya and the Tibetan Plateau.

Early Jurassic high-K calc-alkaline and shoshonitic rocks from the Tongshi intrusive complex, eastern North China Craton: Implication for crust–mantle interaction and post-collisional magmatism

The Tongshi intrusive complex, located within the western Shandong Province (Luxi Block) in the eastern North China Craton, comprises high-K calc-alkaline series (fine-grained quartz monzonite and porphyritic quartz monzonite) and shoshonitic series (coarse- to fine-grained porphyritic syenites). Here we report comprehensive data on petrology, geochemistry, Sr–Nd–Pb isotopes and zircon U–Pb and Hf isotopic compositions from the intrusive complex. LA–ICPMS zircon U–Pb ages show that this complex was emplaced at 180.1–184.7Ma. The fine-grained quartz monzonite and porphyritic quartz monzonite have similar major and trace elements features, implying a similar petrogenetic history. Coupled with the widespread Neoarchean inherited zircons in these rocks, the high SiO2 and Na2O as well as the low MgO contents and low Pb isotopic ratios ((206Pb/204Pb)i=15.850–16.881, (207Pb/204Pb)i=14.932–15.261, (208Pb/204Pb)i=35.564–36.562) of the quartz monzonites suggest an origin from ancient tonalite–trondhjemite–granodiorite (TTG) crust. However, their higher Nd and Hf isotopic ratios (εNd (t)=−11.7 to −7.0, εHf (t)=−25.0 to −10.3) as compared to the basement rocks indicate input of enriched lithospheric mantle-derived materials. The coarse- to fine-grained porphyritic syenites were derived from similar sources as inferred from their comparable major and trace elements contents as well as the Nd, Hf and Pb isotopic compositions. The Neoarchean inherited zircons and depletion of Nb, Ta, P and Ti in these rocks indicate the involvement of ancient crust. However, the high Nd and Hf isotopic ratios (εNd (t)=−0.8 to 1.5, εHf (t)=−4.4 to 4.8) coupled with high Pb isotopic compositions ((206Pb/204Pb)i=18.082–19.560, (207Pb/204Pb)i=15.510–15.730, (208Pb/204Pb)i=37.748–39.498) suggest that the porphyritic syenites were mainly derived from an asthenospheric mantle. Based on the geochemical and isotopic features, a magmatic process similar to MASH (melting, assimilation, storage and homogenization) is proposed for the fine-grained quartz monzonite and AFC (assimilation and fractional crystallization) is envisaged for the porphyritic quartz monzonite and porphyritic syenites. Our data suggest the interaction of various crustal and mantle sources including lower to upper crust, lithospheric mantle and asthenospheric mantle in the generation of the Tongshi intrusive complex. Sharing common characteristics with other post-collisional magmatic rocks, the high-K calc-alkaline and shoshonitic rocks from the Tongshi complex are considered to have originated in post-collisional setting, marking the beginning of lithospheric destruction in the inner domains of the NCC during Mesozoic.