The Tectono-magma Events In The Eastern Margin Of The Himalayan Syntaxis And Their Geodynamic Implications

The Tibetan Plateau has experienced complex tectonic evolution processes such as the subduction of Tethyan oceanic lithosphere (including the Proto-Tethys, Paleo-Tethys, Neo-Tethys) and the following continent-continent collision between Indian and Eurasia plates. Widespread magmatic activities occurred in these geological processes. The Gangdese batholiths, located in the southern Lhasa terrane, resulted from the subduction of the Neo-Tethyan oceanic slab and the India-Eurasia continental collision as well as post-collisional convergence. Therefore, it can provide constraints on the evolution of the Neo-Tethys and the Tibetan uplift. In the paper, author presents field geology, petrography, LA-ICPMS zircon U-Pb geochronology, geochemistry and Sr-Nd-Hf isotopic compositons for the granitoids from the east margin of the eastern Himalayan syntaxis. The studying aims are (1) revealing the petrogenesis of these granitoids; and (2) discussing the tectonic evolution of the eastern Lhasa terrane. The main results are as follows:1. Sixteen granitoid samples and four mafic intrusive samples from the Motuo tectono-magmatic belt were analysed for in situ zircon U-Pb dating. Obtained magma crystallization ages range from-164Ma to-24Ma. Combined with previously published zircon U-Pb dating and geochemical data, the magmatic activities in the the Motuo tectono-magmatic belt can be divided into four intrusive stages:(1)164-125Ma,(2)-83Ma,(3)71-45Ma, and (4)30-24Ma.2. The Middle Jurassic to Early Jurassic granitoids contain relatively low SiO2(61.53-71.44wt.%) with metaluminous to weakly peraluminous (A/CNK=0.92-1.13, mostly<1.08). The Middle Jurassic to Early Jurassic granitoids show high Isr ratios (0.7104-0.7189) and negative whole-rock εNd(t) values (-8.6to-15.5), with Nd two-stage depleted mantle model ages of1.6-2.2Ga. Zircon Hf isotopic data show eHf(t)=-15.0±0.4、-16.2±0.4、-17.0±0.6. These geochemical and Sr-Nd-Hf isotopic data imply that they were derived from partial melting of Paleoproterozoic crustal materials. It is suggested that the calc-alkaline granodiorite (164Ma) resulted from the northward Neo-Tethyan subduction. This arc magmatism is semi-continuously active during Middle Jurassic to Early Cretaceous (164-125Ma).3. The eastern Lhasa terrane had experienced the Late Cretaceous crustal thickening and northeastward migration of arc magmatism during mid-Cretaceous. We attribute the Late Cretaceous magmatism to the flat (low-angle) slab subduction of the Neo-Tethyan ocean slab in the eastern Lhasa terrane during mid-Cretaceous.4. The Late Cretaceous granitoids have less evolved whole-rock Sr-Nd isotopic compositions and zircon Hf isotopic compositions than the Middle Jurassic to Early Cretaceous granitoids. The Late Cretaceous samples have high Sr (917-1372ppm) and La (70-155ppm) and low Y (7.5-22.1ppm) and Yb (0.39-1.53ppm) with high Sr/Y (62-122) and (La/Yb)N (45-129) ratios, showing adakitic geochemical signature. This study suggests that they could be derived from delaminated lower crust. Our new and published geochemical and Sr-Nd isotopic data show Sr/Y ratio and Sm-Nd isotope compositional shifts during this time. Furthermore, geochemical signature of the Late Cretaceous Bolonggong adakitic rocks suggests their magma generation from partial melting of delaminated lower crust. These features imply that delamination of thickened lower crust played an important role in eastern Lhasa terrane during ca.83-70Ma.5. The Paleocene-Eocene samples contain SiO2contents of63.91-73.50wt.%(mostly>68wt.%) with metaluminous to weakly peraluminous (A/CNK=0.97-1.08). The Sr-Nd isotopic compositions [Isr=0.7056to0.7100, εNd(t)=+3.0to-3.1] of these granitoids are comparable to those of the Late Cretaceous adakitic rocks. The large variation in geochemical compositions of the Paleocene-Eocene granitoids is most likely resulted from the variable contributions of the newly underplated mantle materials and the mature crustal materials. The Meiri granodiorite-granite samples were originated from partial melting of the juvenile basaltic lower crust. These magmas have minor assimilation with mature crustal materials during their magma emplacement. In contrast, the Beibeng and52K granites have increasing mature crustal (the Bomi Group) contributions. The Motuo Eocene mafic intrusions are characterized by variable SiO2(44.60-57.60wt.%), high A12O3(17.19-20.86wt.%), low MgO (1.85-5.38wt.%) with Mg#of34-59, comparable to low-MgO high-alumina basalts to high-alumina basaltic andesites. They exhibit variable LILEs, enrichment of LREE and negative anomalies of HFSEs. These mafic rocks display weakly enriched initial whole-rock Sr-Nd and zircon Hf isotope compositions (Isr=0.7065to0.7086,εNd(t)=-3.59to+1.22, and εHf(t)=+0.9to+6.4). Whole-rock geochemical and Sr-Nd isotopic and zircon Hf isotopic data indicate that these mafic intrusions were derived from partial melting of the metasomatised lithospheric mantle. Geochronological data reveal a southward age decreasing from the52K (71Ma), Meiri (56Ma), to Beibeng (45Ma) in the eastern side of the eastern Himalayan syntaxis, implying the geodynamic processes (slab rollback and break-off) is similar to that of the central part of the southern Lhasa terrane.6. The Oligocene two-mica granites in the eastern margin of the east Himalayan syntaxis are high-K calc-alkaline (SiO2=67.3-75.0wt.%, A12O3=13.3-15.8wt.%, K2O/Na2O=0.7-1.9). An outstanding geochemical feature is that they have high Sr/Y ratios (19-88, mostly>30), coupled with low Y and HREE. Sr-Nd isotopic data indicate that they were derived from partial melting of the thickened lower crustal materials. The potential magma sources are the Gangdese Jurassic-Eocene granitoids and basement metasedimentary rocks. The later contribution to their magma sources is increasing from south to north in pluton geographic location. Our data show two decreasing variations for their magma crystallization ages from south to north in the eastern margin of the east Himalayan syntaxis and from east to west along the Gangdese belt. Such variations resulted from diachronological breakoff of subducted Indian continental slab. In the east Himalayan syntaxis, the breakoff of the subducted Indian continental slab commenced in south and then propagated towards north probably due to heat-effect of slab breakoff window widening. Along the Gangdese belt, the breakoff of the subducted Indian continental slab commenced in east, and then propagated towards west7. The Middle Jurassic to Early Cretaceous granitoids in the Motuo tectono-magmatic belt have strongly negative whole-rock εNd(t) values (-15.5to-8.6), with old Nd two-stage depleted mantle model ages (1.6-2.2Ga). These Nd isotopic compositions indicate that the granitoids were derived from anatexis of an ancient basement. The author suggestss that the basement of the Motuo tectono-magmatic belt is the eastern extension of the basement of the central Lhasa terrane rather than the southern Lhasa terrane. Our secular tectonic evolution model shows that the Jurassic to Eocene geodynamic evolution in the eastern Lhasa terrane is roughly comparable with that in central Gangdese belt. Along the central and eastern parts of the southern Lhasa terrane, the Neo-Tethyan ocean slab subduction may simultaneously run during the latest Triassic to Early Jurassic and the collision between the India and Eurasia plates coevally started before55Ma. Besides, Cretaceous flat-slab subduction of the Neo-Tethyan Ocean floor is widespread in this belt. The visible difference between their tectonic settings mainly took place in Late Cretaceous. The initially more mature crust in the eastern Lhasa terrane may be the original cause for these tectonic setting variations along the central and eastern part of the southern Lhasa terrane. We propose that the crust in the eastern Lhasa terrane significantly thickened and formed eclogitized lower crust during Early Cretaceous (ca.125-95Ma) shortening, causing by the Neo-Tethyan flat-slab subduction. Then, Late Cretaceous ridge subduction of the Neo-Tethyan lead the eclogitized lower crust to weak enough to foundering during ca.83-70Ma.8. Zircon Hf isotopic data exhibit three crustal evolution models. The Late Triassic to Late Cretaceous granitoids show depleted zircon Hf isotopic compositions, suggesting origination from partial melting of the juvenile basaltic lower crust. A few mature crustal material incorporated during India-Asia continental collision. The zircon εHf(t) values indicate an increased contribution of amantle component in the generation of the Early Cretaceous igneous rocks in the central and north Lhasa subterrane at-110Ma. The jumping zircon Hf isotopic composition variations between the pre-Early Cretaceous and post-Late Cretaceous magmas in the eastern Lhasa terrane probably resulted from arc root delamination in the eastern Lhasa terrane during Late Cretaceous.