Sedimentary record of Jurassic northward subduction of the Bangong–Nujiang Ocean: insights from detrital zircons

The subduction polarity and related arc–magmatic evolutional history of the Bangong–Nujiang Ocean, which separated the South Qiangtang terrane to the north from the North Lhasa terrane to the south during the Mesozoic, remain debated. This study tries to reconstruct the subduction and evolution of the Bangong–Nujiang Ocean on the basis of U–Pb and Hf isotopic analyses of detrital zircons in samples from sedimentary rocks of the middle-western section of the Bangong–Nujiang suture zone in Gerze County, central Tibet. The Middle Jurassic Muggargangri Group in the Bangong–Nujiang suture zone was deposited in a deep-sea basin setting on an active continental margin. The Late Jurassic strata, such as the Sewa Formation, are widely distributed in the South Qiangtang terrane and represent deposition on a shelf. The Early Cretaceous Shamuluo Formation in the Bangong–Nujiang suture zone unconformably overlies the Muggargangri Group and was probably deposited in a residual marine basin setting. The detrital zircons of the Muggargangri Group contain seven U–Pb age populations: 2.6–2.4 Ga, 1.95–1.75 Ga, 950–900 Ma, 850–800 Ma, 650–550 Ma, 480–420 Ma, and 350–250 Ma, which is similar to the age populations in sedimentary rocks of the South Qiangtang terrane. In addition, the age spectra of the Shamuluo Formation are similar to those of the Muggargangri Group, indicating that both had a northern terrane provenance, which is conformed by the north-to-south palaeocurrent. This provenance indicates northward subduction of the Bangong–Nujiang oceanic crust. In contrast, two samples from the Sewa Formation yield variable age distributions: the lower sample has age populations similar to those of the South Qiangtang terrane, whereas the upper possesses only one age cluster with a peak at ca. 156 Ma. Moreover, the majority of the late Mesozoic detrital zircons are characterized by weakly positive εHf(t) values that are similar to those of magmatic zircons from arc magmatic rocks in the South Qiangtang terrane. The findings, together with information from the record of magmatism, indicate that the earliest prevalent arc magmatism occurred during the Early Jurassic (ca. 185 Ma) and that the principal arc–magmatic stage occurred during the Middle–Late Jurassic (ca. 170–150 Ma). The magmatic gap and scarcity of detrital zircons at ca. 140–130 Ma likely indicate collision between the Qiangtang and Lhasa terranes. The late Early Cretaceous (ca. 125–100 Ma) magmatism on both sides of the Bangong–Nujiang suture zone was probably related to slab break-off or lithospheric delamination after closure of the Bangong–Nujiang Ocean.

中生代时期分隔北侧南羌塘地体（South Qiangtang terrane）与南侧北拉萨地体（North Lhasa terrane）的班公湖-怒江洋（Bangong–Nujiang Ocean）的俯冲极性及其相关弧岩浆演化历史，至今仍存在争议。本研究以藏中部改则县班公湖-怒江缝合带（Bangong–Nujiang suture zone）中-西段沉积岩样品中的碎屑锆石（detrital zircons）为研究对象，通过U-Pb与Hf同位素分析，重建班公湖-怒江洋的俯冲与演化过程。班公湖-怒江缝合带内的中侏罗统木嘎岗日群（Muggargangri Group）形成于活动大陆边缘的深海盆地环境。晚侏罗世地层（如色哇组（Sewa Formation））广泛分布于南羌塘地体，代表了陆棚环境下的沉积。班公湖-怒江缝合带内的早白垩世沙木罗组（Shamuluo Formation）不整合覆于木嘎岗日群之上，其沉积环境可能为残余海盆。木嘎岗日群的碎屑锆石共包含7组U-Pb年龄区间：2.6~2.4 Ga、1.95~1.75 Ga、950~900 Ma、850~800 Ma、650~550 Ma、480~420 Ma以及350~250 Ma，该年龄分布特征与南羌塘地体沉积岩中的年龄分布相似。此外，沙木罗组的年龄谱特征与木嘎岗日群相似，表明二者的物源均来自北侧地体，该结论得到北向南古流向数据的证实。该物源特征指示班公湖-怒江洋壳发生了北向俯冲。与之相反，色哇组的两件样品呈现出截然不同的年龄分布特征：下层样品的年龄区间与南羌塘地体相似，而上层样品仅存在一组年龄峰值，峰值年龄约为156 Ma。此外，绝大多数晚中生代碎屑锆石的εHf(t)值均为弱正值，该特征与南羌塘地体内弧岩浆岩的锆石Hf同位素特征一致。结合岩浆作用记录的相关数据，本研究结果表明：最早的广泛弧岩浆作用发生于早侏罗世（约185 Ma），而主要的弧岩浆活动期则集中于中晚侏罗世（约170~150 Ma）。约140~130 Ma期间的岩浆活动间断以及碎屑锆石的稀缺性，大概率指示了羌塘地体与拉萨地体之间发生了碰撞。班公湖-怒江缝合带两侧的晚早白垩世（约125~100 Ma）岩浆作用，可能与班公湖-怒江洋闭合后的板片断离或岩石圈拆沉作用有关。