Origin And Petrogenesis Of The Post-collisional Magmatism In Both North And South Sides Of Lhasa Terrane

The north and south margins of the Lhasa terrane, where belong to Lhasa-Qiangtang arc-continent(or arc-arc) collision zone and India-Asia continental collision zone, respectively, developed widespread syncollisional to postcollisional magmatism. Thus they are ideal places to study the petrogenesis of postcollisional magmatism and crustal growth of collisional belt. However, the previous studies mainly focused on geochronological and tectonic setting of the magmatic rocks, while the magma source and magmatic process were less studied. Therefore, this study selected the postcollisional magmatic rocks from the two collisional belts in order to reveal their origin and petrogenesis.Zircon U-Pb dating reveals that the Zhuogapu Mg-rich volcanic rocks from Lhasa-Qiangtang collision zone crystallized at ca. 90 Ma, postdating the development of a regional angular unconformity between the Upper Cretaceous and underlying strata. The Zhuogapu volcanic rocks have small negative whole-rock(87Sr/86Sr)i of 0.7054?0.7065 and ?Nd(t) of ?3.2 to ?1.7, positive whole-rock ?Hf(t) of +3.8 ? +7.0 and zircon ?Hf(t) of +5.6 to +8.7. These signatures indicate that the Zhuogapu Mg-rich andesites were most likely derived from partial melting of a delaminated mafic lower crust(including lowermost crust straddling the northern and central Lhasa subterranes) that led to the generation of the Zhuogapu primary melts with adakitic signatures and small negative ?Nd(t). Such melts subsequently experienced interaction of melt-asthenospheric mantle peridotite followed by modification of highly fractionated magmas in shallow crustal magma chamber. Hornblende-controlled fractionation results in the change of geochemical composition from Mg-rich andesitic to Mg-rich dacitic magmas. Field observations, together with geochronological and geochemical data, indicate that the Zhuogapu Mg-rich volcanic rocks and coeval magmatism in the northern Lhasa subterrane may be the result of thickened lithospheric delamination following the final Lhasa–Qiangtang amalgamation.Our data indicate that the Dajia pluton from western Gangdese Batholith of India-Asia continental collision zone consists of moderately and strongly fractionated granites that were emplaced synchronously at ca. 43 Ma. The Group 1 samples have Si O2 contents of 69?72 wt.% and vary in terms of the differentiation index(DI = 84?93), displaying low HREEs and Y abundances. The Group 2 samples are characterized by high Si O2(75?78 wt.%) and DI(95?97) and and Ba, Sr, P, and Ti anomalies that are significantly more negative than those of the Group 1 samples. These data indicate that the two groups were derived from a common source region with garnet as a residual mineral phase. The Group 1 samples were most likely derived from partial melting of garnet-bearing amphibolite(rather than eclogite) within the juvenile southern Lhasa crust and mixed with the enriched components from the subducting ancient Indian continental crust and/or the ancient central Lhasa basement. The Group 2 samples are interpreted as the products of extensive fractional crystallization(plagioclase, K-feldspar, biotite, apatite, allanite, titanite, and ilmenite) of the melts represented by the Group 1 samples. The Group 1 samples and the other ca. 43 Ma coeval magmatism documented both in the Gangdese Batholith and in the Tethyan Himalaya can be best interpreted as the final consequences of the magmatic responses to the Neo-Tethyan oceanic slab breakoff. Low HREEs and Y abundances of the Dajia pluton, together with the presence of strongly fractionated granites(Group 2) identified for the first time in the Gangdese Batholith, indicate that the crust beneath the Dajia region had already been thickened by ca. 43 Ma.Our results also indicate that magmatic zircons from the I-type granitoids are characterized by relatively lower Pb concentrations and higher(Nb/Pb)N ratios, distinct from those of the S-type granitoids that display higher Pb, lower(Nb/Pb)N and significant Eu negative anomalies(Eu/Eu*? 0.3); whereas these values are transitional in zircons from the A-type granitoids. These differences are considered as a useful tool for determining the genetic types of post-collisional granitoids.