Integration of lithostratigraphic, magmatic, and metamorphic data from the LhasaQiangtang collision zone in central Tibet (including the Bangong suture zone and adjacent regions of the Lhasa and Qiangtang terranes) indicates assembly through divergent double sided subduction. This collision zone is characterized by the absence of Early Cretaceous high-grade metamorphic rocks and the presence of extensive magmatism with enhanced mantle contributions at ca. 120110 Ma. Two JurassicCretaceous magmatic arcs are identified from the CaimaDuobuzaRongmaKangqiongAmdo magmatic belt in the western Qiangtang Terrane and from the Along TsoYanhuDaguoBaingoinDaru Tso magmatic belt in the northern Lhasa Terrane. These two magmatic arcs reflect northward and southward subduction of the Bangong Ocean lithosphere, respectively. Available multidisciplinary data reconcile that the Bangong Ocean may have closed during the Late JurassicEarly Cretaceous (most likely ca. 140130 Ma)through arc-arc "soft" collision rather than continent-continent "hard" collision.Subduction zone retreat associated with convergence beneath the Lhasa Terrane may have driven its rifting and separation from the northern margin of Gondwana leading to its accretion within Asia.

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Magnesium isotopic compositions are reported for twenty-four international geological reference materials including igneous, metamorphic and sedimentary rocks, as well as phlogopite and serpentine minerals. The longterm reproducibility of Mg isotopic determination, based on 4-year analyses of olivine and seawater samples, was ≤ 0.07‰ (2s) for d 26 Mg and ≤ 0.05‰ (2s) for d 25 Mg. Accuracy was tested by analysis of synthetic reference materials down to the quoted long-term reproducibility. This comprehensive dataset, plus seawater data produced in the same laboratory, serves as a reference for quality assurance and inter-laboratory comparison of high-precision Mg isotopic data.Significant advances have been made on Mg isotope geochemistry over the past decade. It was debated whether or not the Earth has a chondritic Mg isotopic composition. The most recent studies indicate that the Earth, as well as the Moon, have Mg isotopic composition similar to chondrites within 0.07‰ (2s) in 26 Mg/ 24 Mg ratio (Teng et al.

Eocene magmatic processes and crustal thickening in southern Tibet: Insights from strongly fractionated ca. 43 Ma granites in the western Gangdese Batholith, LITHOS (2015), doi: 10.1016/j.lithos.2015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT

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New geochronological and geochemical data on magmatic activity from the India-Asia collision zone enables recognition of a distinct magmatic flare-up event that we ascribe to slab breakoff. This tie-point in the collisional record can be used to back-date to the time of initial impingement of the Indian continent with the Asian margin. Continental arc magmatism in southern Tibet during 80–40 Ma migrated from south to north and then back to south with significant mantle input at 70–43 Ma. A pronounced flare up in magmatic intensity (including ignimbrite and mafic rock) at ca. 52–51 Ma corresponds to a sudden decrease in the India-Asia convergence rate. Geological and geochemical data are consistent with mantle input controlled by slab rollback from ca. 70 Ma and slab breakoff at ca. 53 Ma. We propose that the slowdown of the Indian plate at ca. 51 Ma is largely the consequence of slab breakoff of the subducting Neo-Tethyan oceanic lithosphere, rather than the onset of the India-Asia collision as traditionally interpreted, implying that the initial India-Asia collision commenced earlier, likely at ca. 55 Ma.