早前寒武纪大陆块壳构造理论——华北克拉通早前寒武纪基底构造格架、地壳演化及其与区域地球化学不均一性的关系研究

早前寒武纪大陆块壳构造理论——华北克拉通早前寒武纪基底构造格架、地壳演化及其与区域地球化学不均一性的关系研究张福勤＊＊（中国科学院地球化学研究所，贵阳５５０００２）关键词大陆块壳并合带地壳演化进程地球化学不均一性早前寒武纪地质学研究是地球科学最基础的...

Constraining quantitatively the timing and process of continent-continent collision using magmatic record： Method and examples

Based on the main driving force of plate motion(the slab pull force generated by the descent of the oceanic plate in subduction zones) and the three primary mechanisms for magma generation(adding fluid, increasing temperature, and decreasing pressure), the continent-continent collisional process has been divided into three stages, including initial collision, ongoing collision, and tectonic transition. These stages are characterized by normal calc-alkaline andesitic magma(dehydration of the oceanic crust to release fluids), the migration of calc-alkaline magma toward the trench(dehydration of the oceanic crust or an increase in temperature) or small-scale crust-derived peraluminous magma(heat from intra-crustal shearing), and extensive magmatism with compositional diversity induced by slab break-off(increasing temperature and decreasing pressure), respectively.On the basis of the obtained age of slab break-off, the timing of the initial continent-continent collision can be quantitatively back-dated using the convergence rate, depth of slab break-off, and subduction angle. The spatio-temporal migration of the magmatic activity of the Gangdese Batholith, the onset of magmatic flare-up, and the increase of magma temperature at 52–51Ma documented by the volcanic rocks of the Linzizong Pana Formation were most likely the result of the break-off of the Yarlung-Zangbo Neo-Tethyan oceanic lithosphere at approximately 53 Ma. This proposed age of slab break-off suggests that the initial India-Asia collision likely occurred at approximately 55–54 Ma, which is consistent with the collision ages constrained by other abundant geological data(60–55 Ma). This magmatic method has been applied to the Bitlis orogenic belt in southern Turkey in the Arabia-Eurasia continental collision zone, yielding an age range of approximately 29–22 Ma for the initial Arabia-Asia continental collision that is close to the collision ages recently obtained by apatite fission-track dating(approximately20 Ma) and regional tectonic shortening(approximately 27 Ma). The intense folding of the Upper Cretaceous and the angular unconformity between the overlying Linzizong volcanic rocks in the southern Lhasa Terrane(90.69 Ma) are not related to the initial continental collision between India and Asia, but can be interpreted as the consequences of the strong coupling between the hot and young subducting oceanic crust immediately south of the spreading ridge and the overriding lithosphere or the subduction of the Neo-Tethys oceanic plateaux or seamounts. The tectonic event documented by the angular unconformity between the Linzizong Dianzhong Formation and the Nianbo Formation lasted approximately 3 Ma and likely marks the initial India-Asia collision. The significant deceleration of the Indian continent at approximately 51 Ma can be attributed to the disappearance of the slab pull force in the subduction zone due to the break-off of the Yarlung-Zangbo Neo-Tethyan oceanic lithosphere. The descent of the eclogitized lower crust of the northern Indian continent provides the main driving force for the current northward motion of Indian plate. The weak deformation of the lithospheric plate in the overriding plate of the India-Asia collisional zone between 60 and 40 Ma can be attributed to the high-angle subduction related to the rollback of the Yarlung-Zangbo Neo-Tethyan oceanic lithosphere after the initial India-Asia continental collision, the presence of the thick crust and high elevation on the southern margin of the Lhasa Terrane, and the decoupling between the mid-upper and lower crust and between the lower crust and lithospheric mantle of the Indian continent.

Initial subduction-related magmatism in southern Alaska identified by geochemistry and zircon Hf-O isotopes

Abundant arc-type magmatic and metamorphic rocks exist on Earth today,which provide insights into the equilibrium state of the subduction process.However,magmatic samples generated during the initial stage of subduction is largely unknown.This hinders our understanding of the subduction initiation process and by inference,the onset of plate tectonics as well as the history of crustal formation.To address this issue,we carried out a comprehensive geochemical-geochronological study of a suite of Late Triassic to mid-Jurassic plutonic rocks from southern Alaska that potentially represent magmas from the initial to mature stages of arc formation.While all studied samples show typical arc-type geochemical signatures,i.e.,enrichment of large ion lithophile elements(LILEs)and depletion of high field strength elements(HFSEs)relative to the heavy rare earth elements(HREEs),the Late Triassic trondhjemites show unique geochemical features such as strongly positiveε_(Hf)(t)andε_(Nd)(t)coupled with lowerδ^(18)O(average 4.77‰±0.09‰).These signatures,along with its higher zircon saturation temperatures compared with younger plutonic rocks,are best explained by shallow partial melting of subducting high-temperature hydrothermally altered lower oceanic crust(i.e.,gabbro).If true,these surprising findings would open up new ways to study subduction initiation which would have important bearing on future research on the onset of global plate tectonics and the formation of the continental crust.