Keda Cai

Zircon U-Pb geochronology and Hf isotopic composition of granitiods in Russian Altai Mountain, Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) consists of many tectonic terranes with distinct origin and complicated evolutionary history. Understanding of individual block is crucial for the reconstruction of the geodynamic history of the gigantic accetionary collage. This study presents zircon U-Pb ages and Hf isotopes for the granitoid rocks in the Russian Altai mountain range (including Gorny Altai, Altai-Mongolian terrane and CTUS suture zone between them), in order to clarify the timing of granitic magmatism, source nature, continental crustal growth and tectonic evolution. Our dating results suggest that granitic magmatism of the Russian Altai mountain range occurred in three major episodes including 445∼429 Ma, 410∼360 Ma and ∼241 Ma. Zircons from these granitoids yield comparable positive εHf(t) values and Neoproterozoic crustal model ages, which favor the interpretation that the juvenile crustal materials produced in the early stage of the CAOB were probably dominant sources for the Paleozoic magmatism in the region. The inference is also supported by widespread occurrence of short-lived juvenile materials including ophiolites, seamount relics and arc assemblages in the northern CAOB. Consequently, the Paleozoic massive granitic rocks maybe do not represent continental crustal growth at the time when they were emplaced, but rather record reworking of relatively juvenile Proterozoic crustal rocks although mantle-derived mafic magma was possibly involved to serve as heat engine during granitic magma generation. The Early Triassic granitic intrusion may be a product in an intra-plate environment, as the case of same type rocks in the adjacent areas. The positive εHf(t) values (1.81∼7.47) and corresponding Hf model ages (0.80∼1.16 Ga) together with evidence of petrology are consistent with the interpretation that the magma source of the Triassic granitic intrusion was derived from enriched mantle and melted under an usually high temperature condition likely due to basaltic magma that underplated the lower crust. Our data combined with evidence of the regional geology enable us to conclude that the Gorny Altai and Altai-Mongolian terranes possibly have similar tectonic natures, but represent two separate accretionary systems before Devonian collision. The accretion and amalgamation processes resulted in the Paleozoic granitoid magmatism and caused the two terranes to merge as a composite tectonic domain at the Siberian continental margin.

Late Paleozoic closure of the Ob-Zaisan Ocean along the Irtysh shear zone (NW China): Implications for arc amalgamation and oroclinal bending in the Central Asian orogenic belt

The largest accretionary orogen in the world, the Central Asian orogenic belt, has evolved through the assembly of various oceanic and continental blocks. Understanding the processes associated with the development of this orogenic belt relies on precise recognition of the boundaries between various terranes. One such major suture zone, which records the collisional history of the Siberian marginal arc system (Chinese Altai) with intra-oceanic arc systems (East/West Junggar), is the Irtysh shear zone in NW China. The spatial continuity and the tectonic nature of this shear zone are still poorly understood, but its development has supposedly made a significant impact on the architecture of the western Central Asian orogenic belt and the formation of the Kazakhstan orocline. Here, we provide new insight into the evolution of this shear zone based on detrital zircon ages, Hf isotope composition, structural data and Ar/Ar age constraints on the timing of deformation. Our results show a major discrepancy in detrital zircon populations and Hf model ages across the southern Chinese Altai and the northern East/West Junggar, thus allowing us to map the exact location of the tectonic boundary. Detrital zircon data constrain the initial closure of the Ob-Zaisan Ocean to the late Carboniferous (<323 Ma), and new structural and Ar/Ar geochronological data shed light on the subsequent collisional processes. We propose that the collisional zone between the Chinese Altai and the East/West Junggar was initially subjected to crustal thickening at ca. 323-295 Ma, followed by orogen-parallel extension (ca. 295 Ma) and transpressional deformation (ca. 286-253 Ma). The closure of the Ob-Zaisan Ocean in NW China postdates the initial phase of oroclinal bending in the western Central Asian orogenic belt, thus indicating that oroclinal bending initiated during subduction. Based on our new constraints and other available geological data, we suggest that the early stage of oroclinal bending was likely driven by slab rollback.

Petrogenesis of the Permian Intermediate-Mafic Dikes in the Chinese Altai, Northwest China: Implication for a Postaccretion Extensional Scenario

The Central Asian Orogenic Belt is a long-lived accretionary orogen, and the late Paleozoic has been considered to be a critical period for the terminal amalgamation of its three tectonic collage systems. However, the exact timing of amalgamation and the geological process of such a huge accretionary orogenic belt are poorly understood. This study presents new geochronological and geochemical data for Permian intermediate-mafic dikes in the Chinese Altai, a key region between the Mongolian and the Kazakhstan collage systems. According to mineral assemblages and petrographic textures, the intermediate-mafic dikes can be categorized as gabbronorite and quartz diorite. The gabbronoritic and quartz dioritic dikes have zircon U-Pb ages of 276.7 ± 2.9 and 273.2 ± 4.3 Ma, respectively. The gabbronorites are characterized by low SiO2 (47.1–51.3 wt%) and high MgO (5.33–8.46 wt%), together with medium Cr (71.2–95.7 ppm) and Ni (80.6–192 ppm) contents. Geochemical modeling indicates that the parental magma was possibly contaminated by 4%–12% crustal materials. Zircon εHf(t) (+13.2 to +16.7) and whole-rock εNd(t) (+4.9 to +6.1) values as well as moderate Sm/Yb ratios (1.75–1.89) imply that the parental magma might have originated from a depleted mantle source dominated by spine lherzolite. In contrast, the quartz diorites exhibit higher SiO2 (57.3–58.3 wt%) and lower whole-rock εNd(t) (∼+2.5) and zircon εHf(t) (+9.1 to +14.4) values, implying that they have a magma source unlike the depleted mantle of the gabbronorites. The parental magma may be derived from mafic lower crust. The quartz diorites have high Y (>39.8 ppm) and heavy rare earth element (e.g., Yb >3.64 ppm) concentrations as well as low Sr/Y (<12) ratios, consistent with geochemical fingerprints of a magma reservoir at shallow depths (<10 kb). Major element compositions of the quartz diorites are comparable to those of intermediate liquids generated by ∼40% partial melting of alkali-enriched basaltic rocks at conditions of T = 1050°–1100°C and P = 8 kbar. Such a high geothermal gradient is inferred to be a consequence of intraplating and/or underplating of hot basaltic magmas in an extensional setting, which may shed light on the ubiquitous tectonic scenario after amalgamation of tectonic collages.

The source and tectonic implications of late Carboniferous–early Permian A-type granites and dikes from the eastern Alataw Mountains, Xinjiang: geochemical and Sr–Nd–Hf isotopic constraints

ABSTRACT Zircon U–Pb ages, and geochemical, Sr–Nd and zircon Hf isotopic compositions are reported for the A-type granites and dikes in the Alataw Mountains of the northwestern Tianshan Orogenic Belt (NTOB), with the aim of investigating the sources and genesis of A-type granites and dikes. The laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating of A-type granites yielded a concordant weighted mean 206Pb/238U age of 297.4 ± 1.5 and 300.6 ± 0.9 Ma, respectively, defining a late Carboniferous–early Permian magmatic event. Geochemically, the granitic intrusions and dikes are characterized by high SiO2 and total alkalies (K2O + Na2O), high Zr, Nb, Ta content, and Ga/Al ratio with prominent negative Ba, Sr, P, Eu, and Ti anomalies. These features indicate that the granitic intrusions and dikes in the eastern Alataw Mountains are of an A-type affinity. The depleted Nd isotope compositions of the granitic intrusions and dikes are consistent with those of the Carboniferous volcanic rocks in the Alataw Mountains, especially Carboniferous adakites (εNd(t) = +3.6 to +6.6), suggesting that they were likely generated by partial melting of less evolved crustal materials, such as oceanic crust stored in the middle and/or lower crust or Carboniferous volcanic arc crust. The widespread late Carboniferous–early Permian magmatism in the NTOB may have been related to a ridge subduction accompanied by slab roll-back of the subducting plate of the North Tianshan Ocean.

Petrogenesis and tectonic implications of early Devonian mafic dike–granite association in the northern West Junggar, NW China

ABSTRACT Mafic dike–granite associations are common in extensional tectonic settings and important and pivotal in reconstructing crust–mantle geodynamic processes. We report results of zircon U–Pb and hornblende 40Ar-39Ar ages and major-element and trace-element data for mafic dike–granite association from the northern West Junggar, in order to constrain their ages, petrogenesis, and geodynamic process. The mafic dike–granite association was emplaced in the early Devonian. The Xiemisitai monzogranites have high SiO2 contents and low MgO, Cr, and Ni concentrations, suggesting that they were mainly derived from crustal sources and were probably generated by partial melt of the juvenile mid-lower crust. The mafic dikes have low Mg# and Cr and Ni abundances, suggesting that they have experienced significant fractional crystallization. The Xiemisitai mafic dikes contain hornblende and biotite and display negative Nb–Ta–Ti anomalies, enrichment of LREEs and LILEs, and depletion of HREEs and HFSEs, consistent with an origin from a lithospheric mantle metasomatized by subducted slab-derived fluids. In addition, the Xiemisitai mafic dikes are plotted within melting trends with little to no garnet (Cpx: Grt = 6:1) in their source. The La/Yb versus Tb/Yb plot also indicates the presence of less than 1% residual garnet in the source region for the Xiemisitai mafic dikes. Therefore, it can be inferred that the Xiemisitai mafic dikes were generated at a correspondingly shallow depth, mostly within the spinel stability field. The Xiemisitai mafic dikes were most probably generated by the partial melting of the metasomatized lithospheric mantle at relatively shallow depths (<80 km). The Xiemisitai mafic dike–granite association could have been triggered by asthenospheric upwelling as a result of the rollback of the subducted Irtysh–Zaysan oceanic lithosphere.