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CRUSTAL COMPOSITIONS VARIATION WITH THE DEPTH IN A MAGMATIC CRUST: AN EXAMPLE FROM THE TILTED GANGDESE BATHOLITH IN SOUTHERN TIBET
The Gangdese Batholith in Tibet represents the magmatic product of the subduction of the Tethyan plate during the Mesozoic-early Cenozoic and the India-Eurasia collision since ~55 Ma. The Gangdese crust (90-94.5°E) is an eastward tilted crustal section in which shallowly emplaced bedrocks (2-3 kbar) are exposed in the west and deeper emplaced bedrocks (10-12 kbar) crop out in the east. Here, we combine geochemical data from the Tibetan Magma Database (Chapman & Ducea, 2017) with bedrock pressure estimates to investigate how magmatic compositions vary with depth. We divide the crust into lower, middle, and upper levels and investigate the magmatic compositions in three temporal groups corresponding to three High-Flux Events (HFEs).
For SiO2 the ~90 Ma HFE, interpreted as a classic continental arc, shows unimodal Kernel Density Estimations (KDEs) for all three crustal levels with the lower crustal plutons peaking at ~55 wt%, the mid crust at ~68%, and the upper crust at ~60%. The ~50 Ma HFE that spans the time of collision shows bimodal KDEs for all three crustal levels with the lower crust having a major peak around ~47% and a minor peak around ~71%. The mid crust is more differentiated than the lower crust with a major peak at ~70% and a minor peak at~ 53%. The upper crust shows a weak bimodal distribution with major peaks at ~54% and ~67%. The youngest HFE at ~20 Ma is dominated by similar unimodal KDEs (peaks at 65-70%) throughout the crust. We find that: (1) the middle crust is always more felsic than the upper crust. The latter includes the volcanics; (2) SiO2 diverges at all crustal depths during the second HFE into increasingly bimodal distributions; (3) felsic magmatism dominates the third HFE.
We hypothesize that mantle melting happens at greater depth after the transition from subduction to collision at 70-40 Ma resulting from (1) upper plate thickening, (2) mantle upwelling due to oceanic slab breakoff, (3) and/or the arc root delamination. Such melting processes generated more mafic magma, while collision also promoted partial melting of the crust and a coeval pulse of more felsic magmatism. By comparing Gangdese crust compositions with other crustal sections as well as estimates for global continental crust we will further test how transitions from subduction to collision contribute to the formation of continental crust.