Magmatic record of India-Asia collision

Abstract

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.

Figure 1. Schematic sequence of the relationship between collisional processes and magmatic responses in collision zones.

This figure is generated by Di-Cheng Zhu, using Adobe Illustrator CS4 created by the Adobe Illustrator Team under an open license.

Figure 2. Tectonic framework of the Tibetan Plateau and the Lhasa Terrane.

(a) Showing the Gangdese arc in the context of the Tibetan Plateau. IYZSZ = Indus-Yarlung Zangbo suture zone, BNSZ = Bangong-Nujiang suture zone, JSSZ = Jinsha suture zone. (b) The distribution of the Linzizong volcanic rocks. (c) The stratigraphic column of the Linzizong volcanic rocks in Linzhou Basin. The filled ovals with numerals are host-rock crystallization ages in Ma using in situ zircon secondary ion mass spectrometry (SIMS) U-Pb dating method (see supplementary Table S1 for sample details). (d) The distribution of intrusive rocks in the Gangdese arc. The filled circles indicate sample locations, numerals within ovals are host-rock crystallization ages in Ma using in situ zircon LA-ICPMS U-Pb dating method (see supplementary Table S2 for sample details). Five groups of zircon ages are recognized on the basis of spatial variation of magmatism and different magmatic origin. This figure is generated by Di-Cheng Zhu, using Adobe Illustrator CS4 created by the Adobe Illustrator Team under an open license. (e) Histogram of crystallization ages (Ma) of the intrusive rocks (85–94°E) from the Gangdese Batholith. The red line represents frequency curve. Age data used in this histogram are the crystallization ages defined by the youngest group of zircon analyses of each sample. The bin width was set at 1.5 Ma to accommodate average age uncertainties of 1.1 Ma (2σ; Table S3). Only one age datum is selected for each pluton if between-sample age difference is lower than 3 Myrs. If this difference is more than 3 Myrs, this pluton is considered to emplace at different pulses and thus the different emplacement ages are used to construct the histogram.

Figure 3. Changes in magmatic compositions with time in the Gangdese arc.

(a,b) Plots of SiO₂ content versus age (Ma) and of zircon saturation temperature (°C) versus age (Ma) for the Linzizong volcanic rocks (see supplementary Table S4 for geochemical data). Zircon saturation temperatures were calculated from whole-rock compositions with SiO₂ >56 wt.% following the method of Watson and Harrison (1983). (c) Plot of SiO₂ content versus age (Ma) for the Gangdese Batholith (E85°–E95°) (see Table S5 for geochemical data). Note that this plot did not show a clear increase of mafic magmatism at ca. 51 Ma as indicated by the presence of well-developed mafic enclaves and dykes (Fig. S1); this inconsistency reflects sampling bias with mafic material underrepresented.

Figure 4. Schematic illustrations showing the India-Asia collisional processes and resultant tectonomagmatic activity over the past 70–40 m.y. (not to scale).

This figure is generated by Di-Cheng Zhu, using Adobe Illustrator CS4 created by the Adobe Illustrator Team under an open license.