Synchronous Oligocene-Miocene metamorphism of the Pamir and the north Himalaya driven by plate-scale dynamics

Abstract

Gneiss domes in the Pamir (Central Asia) and the Himalaya provide key data on mid-to deep-crustal processes operating during the India-Asia collision. Laser ablation split-stream inductively coupled plasma-mass spectrometry (LASS-ICP-MS) data from monazite in these domes yield a time record from U/Th-Pb dates and a petrologic record from rare earth element (REE) abundances. Seven samples from the Pamir and six samples from the north Himalayan gneiss domes yield almost identical monazite dates of ca. 28-15 Ma. Most monazite has invariant heavy REE (HREE) abundances; two samples, however, have older monazite that records progressive HREE depletion and two samples have younger monazite that records progressive HREE enrichment. These variations in HREE are compatible with increasing garnet abundance-prograde metamorphismuntil ca. 20 Ma, and decreasing garnet abundance thereafter. The change from HREE depletion to enrichment may record a transition from crustal thickening and heating to dome exhumation and cooling. This documentation of synchronous Barrovian metamorphism within domes of Indian crust along the margin of the orogen (Himalaya) and within domes of Asian crust within the core of the orogen (Pamir) is best explained by a plate-scale driving force rather than by local events. We propose that widespread, synchronous thickening was initiated by the resumption of Indian subduction following slab breakoff and then terminated by a second slab-tearing event-both plate-scale events inferred from tomography.

Early evolution of the Pamir deep crust from Lu-Hf and U-Pb geochronology and garnet thermometry

“…An inverse correlation between U/Th-Pb age and (Yb/Gd) N for 28-20 Ma monazite from the central Pamir domes suggests equilibration with growing garnet until ca. 20 Ma (Stearns et al, 2013). The seizing of garnet growth at that time was coeval with a change from prograde to retrograde metamorphism (Robinson et al, 2007;Schmidt et al, 2011) and the onset of cooling and exhumation by approximately on May 24, 2015 geology.gsapubs.org Downloaded from north-south crustal extension (21-18 Ma;Stübner et al, 2013aStübner et al, , 2013b.…”

“…The gneiss-cored domes in the Pamir plateau provide a rare window into the deep regions of the mountain belt ( Fig. 1; e.g., Robinson et al, 2004Robinson et al, , 2007Schmidt et al, 2011;Stübner et al, 2013a;Stearns et al, 2013). This study addresses the evolution of various gneiss samples from these domes in space and time, using integrated U-Pb rutile thermochronology, garnet diffusion thermometry, and garnet Lu-Hf geochronology; the latter dating approach was chosen because it typically dates prograde garnet growth and is robust against diffusive reequilibration even during high-temperature overprinting (Scherer et al, 2000).…”

Neogene Source-to-Sink Relations between the Pamir and Tarim Basin: Insights from Stratigraphy, Detrital Zircon Geochronology, and Whole-Rock Geochemistry

“…Subsequently, basin-ward thrusting of the KYTS occurred (Cao et al 2013b), roughly synchronous with the cessation of dextral-slip (Sobel et al 2011), which may reflect a kinematic transition of this fault from strike-slip to reverse deformation. These tectonic events, coeval with doming of the Muztaghata massif (Robinson et al 2007;Sobel et al 2011;Cao et al 2013b;Thiede et al 2013), indicate crustal contraction advanced to the eastern Pamir during the middle-late Miocene, possibly related to continued subduction of the Indian crust (Stearns et al 2013). This may have induced crustal contraction and thickening of the eastern Pamir and driven thrusting of Pamir crust over the Tarim lithosphere, as imaged by seismicity and gravity data (Lyon-Caen and Molnar 1984; Kao et al 2001;Wittlinger et al 2004).…”

“…Many of the intrusive and volcanic rocks in the central and southeast Pamir have late Eoceneearly Oligocene ages (Budanov et al 1999;Ratschbacher personal communication), which are likely the sources of ∼40 Ma igneous-origin zircons in modern river sediments in the western Pamir (Lukens et al 2012) and Cenozoic deposits at Oytag in the western Tarim Basin (Bershaw et al 2012;Sun and Jiang 2013). Migmatization and leucogranite crystallization that occurred at ∼22-19 Ma (Stü bner et al 2013a(Stü bner et al , 2013b, synchronous with high-temperature metamorphism in the central and south Pamir terranes (Stearns et al 2013), indicate considerable crustal thickening beneath the Pamir during subduction of the Indian plate (Schmidt et al 2011;Stearns et al 2013). In the eastern Pamir, the Tashkurgan alkaline complex emplaced at ∼11 Ma (Robinson et al 2007;Ke et al 2008;Jiang et al 2012), coeval with the eruption of Tajikistan volcanic rocks to the southwest (Ducea et al 2003;Hacker et al 2005).…”

“…1). Most of them have undergone Barrovian metamorphism within the crust between 30-40 km deep since Oligo-Miocene time (Hubbard et al 1999;Robinson et al 2004Robinson et al , 2007Schmidt et al 2011;Stearns et al 2013) during the convergence of India and Asia, followed by widespread denudation at 21-13 Ma (Lukens et al 2012). In the southwestern Pamir, the doming of the Shakhdara massif initiated along its northern boundary at 21-20 Ma and continued along its southern margin from ∼18-15 Ma to ∼2 Ma (Stü bner et al 2013b).The domes in the eastern Pamir are bound by the Kongur Shan extensional system ( fig.…”