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•Zircon U–Pb dating indicates the emplacement of Ruocuo adakitic porphyries during the Early–Middle Jurassic (182–170 Ma).•Ruocuo adakitic porphyries were generated by the partial ...melting of the Neo-Tethys oceanic slab.•Formation of Early Mesozoic magmatic rocks in the southern Lhasa subterrane in a magmatic arc setting.•The northern Xigaze forearc basin has a potential for subduction-related porphyry Cu–Au mineralization.
Jurassic adakitic rocks in the southern Lhasa subterrane are relatively rare, poorly documented, and their petrogenesis and geodynamic setting are unclear. To explore these issues, we present zircon U–Pb ages along with Hf isotopic, whole-rock geochemical, and Sr–Nd–Pb isotopic data for hornblende quartz diorite porphyry (HQDP) and quartz diorite porphyry (QDP) in the Ruocuo area, southern margin of the Lhasa terrane, Tibet. Zircon U–Pb dating for these rocks indicates that they were emplaced in the Early–Middle Jurassic (182–170 Ma). Geochemically, HQDP and QDP both show characteristics of adakites, with intermediate SiO2 (60.62–65.63 wt% and 57.46–60.04 wt%, respectively), relatively high Al2O3 (14.23–18.23 wt% and 18.32–20.14 wt%, respectively) and Sr (344–571 ppm and 514–614 ppm, respectively), and low Y (9.12–13.0 ppm and 11.0–14.9 ppm, respectively) and Yb (0.93–1.31 ppm and 1.08–1.52 ppm, respectively). In addition, they show relatively low ratios of (87Sr/86Sr)i (0.7038–0.7045), (206Pb/204Pb)i (18.31–18.42), (207Pb/204Pb)i (15.56–15.66), and (208Pb/204Pb)i (38.36–38.77) and relatively high values of εNd(t) (+5.54 to +6.33) and εHf(t) (+11.8 to +17.3). The Sr–Nd–Pb–Hf isotopes of HQDP and QDP are similar to those of the Yarlung Tsangpo ophiolites, indicating that they were predominantly generated by the partial melting of the subducted Neo-Tethys oceanic slab. The subducted Neo-Tethys oceanic slab-derived adakitic rocks that are present in the southern Lhasa subterrane provide strong evidence for proving the presence of oceanic subduction during the Early–Middle Jurassic. By combining our results with the previously reported results, we suggest that the Ruocuo adakitic rocks and Early Mesozoic (Late Triassic to Middle Jurassic) magmatic rocks in the southern Lhasa subterrane formed in a magmatic arc setting related to the northward subduction of the Neo-Tethys oceanic slab, that the initial subduction of the Neo-Tethys oceanic slab occurred prior to the Early Jurassic, and that the upwelling asthenosphere, triggered by the roll-back of the subducted Neo-Tethys oceanic slab, provided the heat that was required for slab melting. However, contrasting these results with the geochemical characteristics of the No. 2 deposit in the Xiongcun Cu–Au district leads us to consider that the Early–Middle Jurassic subducted oceanic slab-derived adakitic porphyries contain a greater potential for porphyry Cu–Au mineralization than the Early Mesozoic normal magmatic rocks in the southern Lhasa subterrane. Furthermore, mineralized Jurassic porphyry has been reported in the Ruocuo area, adjacent to the northern Xigaze forearc basin, indicating that the northern Xigaze forearc basin also has the potential for subduction-related porphyry Cu–Au mineralization.
An integrated study of comprehensive geochronological, geochemical, and Sr-Nd-Hf isotopic data was undertaken for the A-type Topcam pluton that intruded within the Sakarya Zone (NE Turkey) with the ...aims of elucidating its origin and tectonic significance and gaining new insights into the generation of aluminous A-type granites. New LA-ICP-MS zircon U-Pb crystallization ages of 72 and 73Ma indicate emplacement in the Late Cretaceous time, just after extensive metaluminous I-type magmatism in the area. The pluton consists mainly of alkali feldspar, quartz, plagioclase, amphibole, and biotite with accessory minerals such as magnetite, apatite, and zircon. The outcrop is composed of granite, syenite, monzonite, and quartz monzonite and possesses a wide range of SiO2 content (57–70wt%) with elevated Ga/Al ratios and low Mg# (mostly <43). The pluton is metaluminous to weakly peraluminous, with aluminium saturation index (ASI) (molar Al2O3/CaO+K2O+Na2O) values of 0.82 to 1.18, and belongs to the shoshonitic and ultra-potassic series. All the samples exhibit relative enrichment in light rare earth elements (LREE) and significant negative Eu (Eu/Eu*=0.31 to 0.86) anomalies on the chondrite-normalized REE diagram. The rocks are enriched in some large ion lithophile elements (e.g., Rb, Th and Ba), and spidergrams show a relative depletion in Nb, Ti, and Sr. The granitic rocks of the pluton have identical 87Sr/86Sr(i) ratios ranging from 0.70518 to 0.70716, relatively low εNd (t) values varying from −5.5 to −0.4, and TDM ages (0.82–1.19Ga). In situ zircon analyses show that the rocks have variable negative and positive εHf (t) values (−5.5 to 5.9) and Hf two-stage model ages (742 to 1468Ma), which are indicative of minor addition of juvenile material. Sr-Nd isotope modelling suggests mixing of 70–90% of lower crustal-derived melt with ~10–30% of mantle-derived melt at lower crust depths. The heat source for partial melting is provided by upwelling of hot asthenosphere triggered by slab roll-back events. Geochemical and isotopic data reveal that metaluminous A2-type granites were derived from partial melting of the Paleozoic lower continental crust dominated by mafic rocks in amphibolitic composition, with minor input of subcontinental lithospheric mantle-derived magma followed by subsequent limited fractional crystallization to generate a variety of rock types. From integrating all available data with the regional tectonic evolution in the Sakarya Zone and adjacent regions, we attribute generation of aluminous A2-type granites to a back-arc extension in the subduction zone, which is induced by the roll-back of the Neo-Tethyan oceanic slab around 72Ma. Consequently, we conclude that these A-type granites were related to intensive extension tectonic, which peaked during the late Cretaceous (Maastrichtian) in response to the roll-back of the Neo-Tethyan oceanic slab, which is indicative of the final-stage subduction-related magmatism in the Sakarya Zone.
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•The Topcam pluton emplaced in latest Cretaceous in the Sakarya Zone•We present the first discovery of A2-type magmatism in the region•Sr-Nd isotope modeling show that the pluton has mixed origin•The pluton formed in back-arc extensional stage as consequence of slab roll-back
Paleozoic magmatic rocks are widespread in the western Middle Tianshan. Their petrogenesis can provide important insights into the geodynamic evolution of the southwestern Altaids. Here, we present ...an integrated study of U–Pb zircon geochronology and geochemical and Lu–Hf isotopic compositions for the Late Paleozoic shoshonitic Chorukhdairon pluton and genetically and spatially related quartz porphyry in the southern Chatkal–Kurama terrane, western Middle Tianshan. The Chorukhdairon pluton mainly comprises monzodiorite and quartz monzodiorite (first phase), quartz monzonite (second and main phase), monzogranite (third phase), and leucomonzogranite (fourth phase). LA–ICP–MS zircon dating yielded magma crystallization ages of 294–291 Ma and 286 Ma for the Chorukhdairon pluton and quartz porphyry, respectively. All the rocks possess high K
2
O content (3.29–5.90 wt.%) and show an affinity with shoshonite series rocks. They display similar trace element compositions characterized by the enrichment of large ion lithophile elements (e.g., Rb, Th, U, and K) and depletion of high-field strength elements (e.g., Nb, Ta, P, and Ti), compatible with typical arc magmatism. Combined with zircon Lu–Hf isotopic data, we suggest that the Chorukhdairon pluton was produced by partial melting of the enriched mantle, followed by fractional crystallization of pyroxenes, amphibole, plagioclase, biotite, and accessory Fe–Ti oxides, apatite, and zircon. The quartz porphyries are similar to highly fractionated I-type granitic rocks, and their parental magma could result from the mixing of different batches of mantle-derived magmas or magmas derived from the mantle and juvenile lower crust. Considering the continuousness of the Middle Carboniferous to Early Permian magmatism in the western Middle Tianshan and other regional geological data, we suggest that the Chorukhdairon pluton and related quartz porphyry probably formed in an oceanic subduction setting. Furthermore, the temporal and spatial evolution of the Paleozoic magmatism imply that the flat-slab subduction that was induced by the subduction/accretion of seamounts probably occurred beneath the Middle Tianshan during the Middle Devonian to Early Carboniferous, after which the southeastward slab roll-back occurred during the Middle Carboniferous to Early Permian. The late slab roll-back was responsible for the southeastward arc magmatism migration and magmatic flare-up in the Chatkal–Kurama terrane, western Tianshan, and led to the formation of arc-related extensional basins and significant crustal growth in the southwestern Altaids.
Late Silurian–early Devonian magmatism of the NW Junggar region in the Central Asian Orogenic Belt provides a critical geological record that is important for unraveling regional tectonic history and ...constraining geodynamic processes. In this study, we report results of Zircon U–Pb ages and systematic geochemical data for late Silurian–early Devonian largely granitic rocks in NW Junggar, aiming to constrain their emplacement ages, origin and geodynamic significance. The magmatism consists of a variety of mafic to felsic intrusions and volcanic rocks, e.g. adakitic granodiorite, K-feldspar granite, syenitic granite, gabbro and rhyrolite. U–Pb zircon ages suggest that the granitoids and gabbros were emplaced in the late Silurian–early Devonian (420–405Ma). Adakitic granodiorites are calc-alkaline, characterized by high Sr (407–532ppm), low Y (12.2–14.7ppm), Yb (1.53–1.77ppm), Cr (mostly <8.00ppm), Co (mostly <11.0ppm) and Ni (mostly <4.10ppm) and relatively high Sr/Y (31–42) ratios, analogous to those of modern adakites. K-feldspar granites and rhyolites are characterized by alkali- and Fe-enriched, with high Zr, Nb and Ga/Al ratios, geochemically similar to those of A-type granites. Syenitic granites show high alkaline (Na2O+K2O=8.39–9.34wt.%) contents, low Fe# values (0.73–0.80) and are weakly peraluminous (A/CNK=1.00–1.07). Gabbros are characterized by low MgO (6.86–7.15wt.%), Mg# (52–53), Cr (124–133ppm) and Ni (84.7–86.6ppm) contents. The geochemical characteristics of the gabbroic samples show affinity to both MORB- and arc-like sources. All granitoids have positive εNd(t) (+3.9 to +6.9) and zircon εHf(t) (+9.8 to +15.2) values and low initial 87Sr/86Sr ratios (0.7035–0.7043), with young TDM(Nd) (605–791Ma) and TDM(Hf) (425–773Ma) ages, suggesting significant addition of juvenile material. The adakitic granodiorites probably resulted from partial melting of mafic lower crust, leaving an amphibolite and garnet residue. The K-feldspar granites, rhyolites and syenitic granites probably formed from partial melting of the Xiemisitai mid-lower crust, while the gabbroic intrusion was probably generated by interactions between asthenospheric and metasomatized lithospheric mantle. Voluminous plutons of various types (adakites, A-type granites, I-type granites, and gabbros) formed during 420–405Ma, and their isotopic data suggest significant additions of juvenile material. We propose that a slab roll-back model can account for the 420–405Ma magmatic “flare up” in NW Junggar as well as an extensional setting.
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•The intrusions in NW Junggar were emplaced during 420–405Ma.•The intrusions were likely formed by partial melting of mid-lower crust.•A slab roll-back was probably existed in late Silurian–early Devonian in NW Junggar.
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•A series of tholeiitic and alkaline basaltic sills are encountered in the Sydney Basin.•These intrusions—the Avon Sills—yield ages of 202.77 ± 0.68 Ma and 202 ± 7 Ma.•Avon Sills were ...derived from a deep, garnet-bearing source.•Late Triassic magmatism is best explained by lithospheric delamination.
The recognition of lithospheric delamination as a mechanism for magmatism and uplift is under-recognized in the geological record. A pertinent example is the terminal phase of the New England Orogen in eastern Australia, where current explanations of slab roll-back-driven extension are incompatible with plate motions in the Late Triassic. Although abundant mafic rocks are present, almost all Late Triassic temporal information is from felsic rocks. To investigate potential Late Triassic mafic magmatism in the New England Orogen, we date a series of tholeiitic and alkaline mafic products in its back-arc (Sydney Basin) using plagioclase 40Ar/39Ar and apatite U-Pb geochronology. We obtained a plagioclase 40Ar/39Ar plateau age of 202.77 ± 0.68 Ma (2σ) from tholeiitic magmatic products and an apatite U-Pb age of 202 ± 7 Ma (2σ) from a proximal alkaline sill some 70 m deeper, both of which overlap within uncertainty. Complementary trace element geochemistry shows that the tholeiitic and alkaline magmatic products were derived from a similar deep, garnet-bearing source, which we attribute to upwelling asthenosphere underneath a thickened lithosphere. Our data suggest that extension occurred ∼10 m.y. later in the back-arc basin than along its arc, further supporting the notion that slab roll-back could not have caused this terminal phase of Late Triassic extension in the New England Orogen. The Late Triassic magmatism in the New England Orogen is best explained by lithospheric delamination as it accounts for the orogenic architecture, chemical signature of the ca. 200 Ma products and spatio-temporal distribution of Late Triassic magmatic products.
Interactions between subduction dynamics and magma genesis have been intensely investigated, resulting in several conceptual models derived from geological, geochemical and geophysical data. To ...provide physico-chemical constraints on these conceptual models, self-consistent numerical simulations containing testable thermo-mechanical parameters are required, especially considering the three-dimensional (3D) natural complexity of subduction systems. Here, we use a 3D high-resolution petrological and thermo-mechanical numerical model to quantify the relative contribution of oceanic and continental subduction/collision, slab roll-back and tearing to magma genesis and transport processes. Our modeling results suggest that the space and time distribution and composition of magmas in the overriding plate is controlled by the 3D slab dynamics and related asthenospheric flow. Moreover, the decrease of the bulk lithospheric strength induced by mantle- and crust-derived magmas promotes the propagation of strike-slip and extensional fault zones through the overriding crust as response to slab roll-back and continental collision. Reduction of the lithosphere/asthenosphere rheological contrast by lithospheric weakening also favors the transmission of velocities from the flowing mantle to the crust. Similarities between our modeling results and the late Cenozoic tectonic and magmatic evolution across the eastern Mediterranean region suggest an efficient control of mantle flow on the magmatic activity in this region, which in turn promotes lithospheric deformation by mantle drag via melt-induced weakening effects.
•High-resolution 3D petrological and thermo-mechanical numerical model of subduction.•Space–time evolution of magmatism controlled by slab dynamics and related mantle flow.•Melt-induced weakening effects promote lithospheric deformation by mantle drag.•Eastern Mediterranean as a natural test-case for tectonics–magmatism interactions.
•The Qunjisayi rhyolites and granites were formed at 306Ma and 296Ma respectively.•The rhyolites were resulted from melting of granitic source at shallow depth.•The granites were derived from ...juvenile basaltic crustal source.•The geodynamic transformation of the western Tianshan occurred in Early Permian.
A-type magmatism is an effective lithoprobe for constraining the tectonic evolution of orogenic belts. In this study, we have identified two episodes of Late Paleozoic A-type magmatism in Qunjisayi, western Tianshan. Zircon U–Pb geochronology has yielded Late Carboniferous (rhyolites, 306±2Ma)- and Early Permian (granites, 296±3Ma) ages. Both the Qunjisayi rhyolites and granites have depleted Sr–Nd–Hf isotopic compositions, moderate A/CNK ratios (0.91–1) and negative Nb and Ta anomalies. They are also characterized by high K2O+Na2O (8.72–10.6wt.%), FeOT/(FeOT+MgO) (0.86–0.99) and Y/Nb (1.42–2.34) values, similar to those of typical A2-type granitoids. The Qunjisayi rhyolites may have been formed by partial melting of a juvenile calc-alkaline granitic crustal source induced by subduction roll-back in shallow depth. The succeeding Qunjisayi granites were likely to be derived from a lower crustal source that contains depleted mantle-derived components. Asthenospheric mantle upwelling, triggered by post-subduction slab break off, may have been important in the Qunjisayi granites formation. We conclude that the Late Paleozoic Qunjisayi A-type magmatism was genetically linked to the geodynamic transformation from late subduction to collision.
Rapid Mesozoic–Early Cenozoic crustal growth in the Gangdese area, southern Tibet, has commonly been attributed to pre-collisional and syn-collisional underplating of mantle-derived magmas. Here, we ...report on adakitic magnesian charnockites (i.e., hypersthene-bearing diorites and granodiorites) near Milin, in eastern Gangdese, that provide new insights into the crustal growth process of the region. Zircon U–Pb analyses of seven charnockite samples indicate that they were generated in the Late Cretaceous (100–89Ma). They exhibit variable SiO2 (53.9 to 65.7wt.%) contents, high Na2O/K2O (1.6 to 14.4) and Sr/Y (27.2 to 138.7) ratios, low Y (6.5 to 18.5ppm), heavy rare earth element (e.g., Yb=0.6 to 1.6ppm) and Th (0.20–2.39ppm) contents and Th/La (0.02–0.23) ratios, with relatively high Mg# (46 to 56) and MgO (2.0 to 4.5wt.%) values. They are characterized isotopically by high and slightly variable εNd(t) (+2.4 to +4.0) and εHf(t) (+10.1 to +15.8) values with relatively low and consistent (87Sr/86Sr)i (0.7042 to 0.7043) ratios. Their pyroxenes have high crystallization temperatures (876 to 949°C). The Milin charnockites were most probably produced by partial melting of subducted Neo-Tethyan oceanic crust that was followed by adakitic melt–mantle interaction, minor crustal assimilation and fractional crystallization of amphibole+plagioclase. The upwelling asthenosphere, triggered by the roll-back of subducted Neo-Tethyan oceanic lithosphere, provided the heat for slab melting. Therefore, we suggest that, in addition to pre-collisional and syn-collisional underplating of mantle-derived magmas, the recycling of subducted oceanic crust has also played an important role in continental crustal growth in southern Tibet.
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•100–89Ma adakitic magnesian charnockites in Minlin, eastern Gangdese•Adakitic rocks with high εNd(t) (+2.4 to +4.0) and εHf(t) (+10.1 to +15.8) values•Partial melting of subducted Neo-Tethyan oceanic crust during slab roll-back•Recycling of subducted oceanic crust for crustal growth in southern Tibet
Purpose In cruciate retaining total knee arthroplasty, posterior cruciate ligament damage may occur during tibial cutting. A prospective randomized study was conducted to investigate whether a novel ...tibial cutting technique was more effective than the currently used techniques.
Materials and methods Patients undergoing cruciate retaining total knee arthroplasty were recruited in a prospective, randomized, controlled trial. In 25 patients (group 1) the tibial cut was performed using a double tibial cut technique; in 25 (group 2) and 25 (group 3) patients, the bone island and en bloc resection techniques were performed, respectively. Posterior cruciate ligament integrity and femoral rollback were assessed at the end of surgery. The Oxford Knee Score, WOMAC score and range of motion were assessed postoperatively.
Results Posterior cruciate ligament was completely preserved in 92% of patients in group 1 and in 64% in group 2 and 3, respectively (p = 0.03). The Oxford Knee Score and WOMAC scores did not differ between groups (p = 0.4). The mean knee flexion was 126.4°, 121.5° and 123.9° in groups 1, 2 and 3, respectively (p = 0.04). The femoral rollback at 120° flexion was 80.7%, 72.2% and 75.4% in groups 1, 2 and 3, respectively (p = 0.01).
Conclusions The double cut technique preserves the posterior cruciate ligament at significantly higher rates than the bone island or en bloc resection techniques. Better posterior cruciate ligament preservation may improve the femoral rollback and knee flexion.
Level of evidence Prospective randomized controlled trial, Level I.
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