Bao-Fu Han

Age, geochemistry, and tectonic implications of a late Paleozoic stitching pluton in the North Tian Shan suture zone, western China

The Central Asian orogenic belt is the largest tectonic assembly of continental and oceanic terranes on Earth due to closure of the paleo–Asian Ocean in the Phanerozoic. Among major suture zones in the North Xinjiang region of western China, the North Tian Shan suture zone, because of collision between the Yili terrane in the south and the Junggar terrane in the north, contains the youngest ophiolitic rocks and may represent the terminal stage of development of the Central Asian orogenic belt in western China, but the timing of the suture zone remains poorly constrained. A sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb age of 316 ± 3 Ma (i.e., the beginning of the late Carboniferous) from the undeformed Sikeshu pluton, which crosscuts the suture zone, places a crucial upperage bound for the time of collision between the Yili and Junggar terranes. This event occurred later than, or nearly concurrent with, other accretion-collision events in the North Xinjiang region, implying that fi nal terrane amalgamation was completed in the late Carboniferous. The Sikeshu pluton shares geochemical characteristics of the widespread late Carboniferous to Permian postcollisional A-type and I-type granitoids with depleted-mantle–like Sr-Nd isotopic signatures in the North Xinjiang region. They all occurred during a protracted (ca. 320–270 Ma) episode of postcollisional magmatism that may have been induced by basaltic under plating due to either slab breakoff or delamination of thickened mantle lithosphere beneath the Central Asian orogenic belt. The same postcollisional magmatism also generated Cu-Ni-sulfi de– bearing, mafi c-ultramafi c magmatic complexes, adakites, and porphyry-type coppermolybdenum–bearing magmatic rocks in the North Xinjiang region.

Timing, Petrogenesis, and Setting of Paleozoic Synorogenic Intrusions from the Altai Mountains, Northwest China: Implications for the Tectonic Evolution of an Accretionary Orogen

The Altai Mountains are a key area for understanding the development of the Altai Tectonic Collage and accretionary orogen. However, the orogenic processes, particularly their early stage, have not been well understood. In this work, we undertake zircon U‐Pb dating of six Paleozoic synorogenic plutons in order to better define the early magmatic and tectonic evolution of the Chinese Altai Mountains. The results revealed three Paleozoic granitic plutonic events at ca. 460, 408, and 375 Ma. These ages, along with the structural patterns of the plutons, suggest two periods of regional deformation, 460–410 Ma and 410–370 Ma. The granitoids mainly follow the tholeiitic and calc‐alkaline trends and are mostly I type. Sr‐Nd isotopic analyses indicate that the sources of the granitoids contain both old continental and younger (juvenile) mantle‐derived components. Chemical, isotopic, and structural features suggest that the plutons were formed mainly in continental arc settings and that the subduction and accretion processes began at ca. 460 Ma and culminated at ca. 408 Ma. Thus, the Altai orogen was mainly built up during early‐middle Paleozoic time, rather than during late Paleozoic time. Furthermore, the southern Altai terrane comprises not only Silurian to Devonian island arcs but also old continental fragments. With these new constraints, we present a new model to account for the tectonic evolution of the Altai orogen. This model proposes that early‐middle Paleozoic Altai orogenic processes could have experienced formation of an active continental margin, the splitting of this margin to form a back‐arc oceanic basin, and the final closing of the back‐arc basin. Consequently, the opening and closure of back‐arc basins along active margins is probably a common process in the central Asian accretionary orogen.

Trace element and Nd–Sr isotope constraints on origin of the Chifeng flood basalts, North China

Abstract The Chifeng flood basalt field is a component part of the Neogene basalt belt in North China. The basalts erupted on the northern edge of the North China craton during the Miocene and are subalkaline tholeiites in composition, with normative olivine and quartz. They show a limited variation in SiO2 contents, ranging from 48.47 to 51.61 wt.%, and evolved nature as indicated by both Mg#=47.4–65.2 and large variations in Ni and Cr contents. They share the geochemical characteristics of the High-Ti group of the Gondwana continental flood basalts, with TiO2=2.10–2.48 wt.%, Ti/Y=399–584, Zr/Y=4.35–7.21, and Sr>299 ppm, being very similar to the Paranapanema High-Ti basalts, Brazil. In contrast to the Paranapanema lavas which generally have large negative Nb, Sr, and P anomalies and negative eNd values, the Chifeng basalts have no striking Sr and P anomalies, with variable Nb anomalies and positive eNd values. The Chifeng basalts are characterised by low Rb/Ba and Rb/Sr, high Ti/Y, Ti/Yb, and Ba/Y, and variable but high Nb/La ratios, and show progressive enrichment of K2O, P2O5, Nb, Ba, Sr, and Zr with decreasing eNd values, which preclude the possibility that the magmas were contaminated by crustal materials. Their 87 Sr / 86 Sr ratios and eNd values are in the ranges of 0.7039 to 0.7052 and of +1.6 to +4.4, respectively, showing more restricted variations compared with those of the mantle xenoliths from the Hannuoba basalt field. Nd–Sr isotopic compositions and the trace element ratios such as La/Nb, K/Nb, Rb/Nb, Ba/Nb, and Zr/Nb show the OIB-like geochemical signatures. The Chifeng basalts were derived from a heterogeneous source region in the asthenosphere, with little interaction with lithospheric mantle. It is likely that a combination of varied degrees of partial melting with the chemical heterogeneity of a source region could result in negative correlation arrays of eNd vs. K2O, P2O5, Rb, Sr, Ba, Nb, Zr, Ti/Yb, Ba/Y, and La/Sm, in which relatively low LIL and HFS element contents and ratios and high eNd values would be produced by the relatively high degrees of partial melting of the heterogeneous source region.

Late Miocene Thermal Evolution of the Eastern Himalayan Syntaxis as Constrained by Biotite 40Ar/39Ar Thermochronology

We conducted biotite 40Ar/39Ar thermochronological research in the eastern Himalayan syntaxis and the neighboring Mesozoic and early Cenozoic Gangdese batholith belt with the aim of exploring the overall cooling pattern and thermal evolution of this remote region in the Himalayan orogen. A compilation of our newly acquired ages with existing data reveals a new temporal-spatial pattern. First, a temporal gap at 13–7 Ma exists in the cooling history of the study area, with ages <7 Ma within the syntaxis and ages >13 Ma in the Gangdese belt. The gap could be a manifestation of a renewed rapid cooling of the syntaxis since ∼7 Ma within a general region of slow cooling. Second, the isochron contours of the cooling ages have an annulus shape, with a younging trend toward the Namche Barwa–Gyala Peri peaks at the core of the syntaxis at 7–4 and 4–2 Ma and along the margins of the syntaxis at 2–1 Ma. This pattern may be a result of a regionally progressive cooling from the margin to the core of the syntaxis that was punctuated by locally focused accelerated cooling and exhumation. Our new findings imply that the Cenozoic faults bounding the syntaxis controlled the development of the syntaxis. Additionally, new estimates of the exhumati rate suggest that the development of the syntaxis has been accelerating since 7 Ma.

Sr–Nd isotopic characteristics of the Late Cretaceous Shuangyashan suite: evidence for enriched mantle 2 in Northeast China

In Northeast China, large volumes of Mesozoic–Cenozoic igneous rocks have developed as a result of long-lasting subduction of the palaeo-Pacific and Pacific plates beneath the eastern Eurasian continent. Previous studies have convincingly confirmed the presence of depleted mantle (DM), FOcal ZOne (FOZO) mantle and enriched mantle 1 (EM1) end-members; the enriched mantle 2 (EM2) end-member is probably present but it has been poorly constrained. The Late Cretaceous Shuangyashan suite, comprising a monzogabbro and diorite–porphyrite stocks and their cumulate hornblendite enclaves, from the Shuangyashan coal basin, Northeast China, is characterized by high initial 87 Sr/ 86 Sr (0.70922–0.71095) and low initial 143 Nd/ 144 Nd ratios (0.51221–0.51238) at 98 Ma. Their occurrence demonstrates that EM2 is present in the lithospheric mantle of Northeast China and its formation may be related to recycled continental material in a subduction setting.