Birth, life, and demise of the Andean–syn-collisional Gissar arc: Late Paleozoic tectono-magmatic-metamorphic evolution of the southwestern Tian Shan, Tajikistan


Worthington, James R.
Kapp, Paul
Minaev, Vladislav
Chapman, James B.
Mazdab, Frank K.; Ducea, Mihai N.; Oimahmadov, Ilhomjon; and Gadoev, Mustafo

The amalgamation of the Central Asian Orogenic Belt in the southwestern Tian Shan in Tajikistan is represented by tectono-magmatic-metamorphic processes that accompanied late Paleozoic ocean closure and collision between the Karakum-Tarim and Kazakh-Kyrgyz terranes. Integrated U-Pb geochronology, thermobarometry, pseudosection modeling, and Hf geochemistry constrain the timing and petro-tectonic nature of these processes. The Gissar batholith and the Garm massif represent an eastward, along-strike increase in paleodepth from upper-batholith (~21–7 km) to arc-root (~36–19 km) levels of the Andean–syn-collisional Gissar arc, which developed from ~323–288 Ma in two stages: (i) Andean, I-type granitoid magmatism from ~323–306 Ma due to northward subduction of the Gissar back-arc ocean basin under the Gissar microcontinent, which was immediately followed by (ii) syn-collisional, I-S-type granitoid magmatism in the Gissar batholith and the Garm massif from ~304–288 Ma due to northward subduction/underthrusting of Karakum marginal-continental crust under the Gissar microcontinent. A rapid isotopic pull-up from ~288–286 Ma signals the onset of juvenile, alkaline-syenitic, post-collisional magmatism by ~280 Ma, which was driven by delamination of the Gissar arclogite root and consequent convective asthenospheric upwelling. Whereas M–HT/LP prograde metamorphism in the Garm massif (650–750°C/6–7 kbar) from ~310–288 Ma was associated with subduction-magma inundation and crustal thickening, HT/LP heating and decompression to peak-metamorphic temperatures (~800–820°C/6–4 kbar) at ~288 ± 6 Ma was driven by the transmission of a post-collisional, mantle-derived heat wave through the Garm-massif crust.

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Figure 1. Paleozoic magmatic rocks of the Tian Shan, modified fromLi et al. [2006]. Regionally significant magmatic bodies colored to emphasize Paleozoic spatiotemporal magmatic patterns. Sutures are indicated with dashed black lines, ages adopted from Burtman [2010]. Plutonic rock bodies are indicated with bright colors, whereas volcanic rock bodies are indicated with corresponding faded colors. Sedimentary rock units (variably metamorphosed and non-metamorphosed “strata”) indicated in gray. Approximate locations of (U)HP/LT and M–HP/LT belts indicated with light-blue stars, (M–)HT/LP complexes indicated with red stars. Carboniferous–Permian plutonic and volcanic rocks are widespread throughout the Tian Shan, indicating widespread magmatism throughout the Tian Shan during the late Paleozoic. Early Paleozoic (Cambrian–Silurian) magmatism is primarily concentrated in the north Tian Shan (also “Issyk-Kul terrane” and “Yili block”). STS, south Tian Shan; MTS, middle Tian Shan; NTS, north Tian Shan; NETS, northeast Tian Shan; o.m., ophiolitic mélange.

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Tectonics, 5 October 2017. DOI: 10.1002/2016TC004285