Multi-stage metamorphism of the South Altyn ultrahigh-pressure metamorphic belt, West China: insights into tectonic evolution from continental subduction to arc–backarc extension

The South Altyn ultrahigh-pressure (UHP) metamorphic belt is claimed to host the deepest subducted continental crust based on the discovery of former stishovite, and thus can provide unique insights into the tectonic evolution from deep continental subduction and exhumation to arc–backarc extension. In this paper, we present detailed studies of petrography, mineral chemistry, phase equilibria modelling and zircon U-Pb dating for three representative samples involving garnet amphibolite (A1531 & A1533) and associated garnet-biotite gneiss (A1534) from the UHP belt. Three phases of metamorphism are inferred for the rocks. The first phase high pressure (HP)–UHP-type eclogite facies is represented by the mineral assemblages of garnet and phengite inclusions in zircon and garnet cores with the high grossular (XGrs = 0.33–0.34). The Si contents of 3.40–3.53 and 3.24–3.25 p.f.u. in phengite inclusions yield pressure conditions of >1.7–2.3 GPa for A1533 and 2.5–2.55 GPa for A1534 at a fixed temperature of 770 °C. The second phase medium-pressure (MP)-type overprinting of garnet amphibolite facies shows P–T conditions of 0.8–1.2 GPa/750–785 °C based on the stability fields of corresponding mineral assemblages, the measured isopleths of Ti contents in biotite and amphibole cores, and XGrs in garnet. The third phase low-pressure (LP) type overprinting includes early-stage heating to peak granulite facies followed by cooling towards a late-stage amphibolite facies. The peak granulite facies is represented by the high Ti amphibole mantle, high Zr titanite and the intergrowths of clinopyroxene + ilmenite in A1533 & A1531, with P–T conditions of 800–875 °C/0.80–0.95 GPa. The late-stage is defined by the solidus assemblages, giving P–T conditions of 0.5–0.7 GPa/720–805 °C. U-Pb geochronology on metamorphic zircons from A1533 and A1534 gives three ages of c. 500 Ma, c. 482 Ma and c. 460 Ma. They are interpreted to represent the HP–UHP, MP and LP types of metamorphism respectively, based on cathodoluminescence images, mineral inclusions and trace element patterns. Combining the regional geology and metamorphic evolution from the Altyn Orogen, a tectonic model is inferred, including the following tectonic scenarios. The small Altyn Microcontinent was subducted to great mantle depths with dragging of the surrounding vast oceanic lithosphere to undergo the HP–UHP eclogite facies metamorphism during the early subduction stage (c. 500 Ma) of the Proto-Tethys Ocean. Then, the subducted slabs were exhumed to a thickened crust region to be overprinted by the MP-type assemblages at c. 482 Ma. Finally, an arc–backarc extension was operated within the thickened crust region due to the retreat of subduction zones. It caused evident heating and the LP-type metamorphic overprinting at c. 460 Ma, with a fairly long interval of 30–40 Myr after the HP–UHP metamorphism, distinct from the short interval of <5–10 Myr in the Bohemian Massif.

Supplemental Material: Eocene thickening without extra heat in a collisional orogenic belt: A record from Eocene metamorphism in mafic dike swarms within the Tethyan Himalaya, southern Tibet

Table S1: EPMA data of minerals compositions for garnet amphibolite; Table S2: Zircon U-Pb SHRIMP data for garnet amphibolite T0526; Table S3: Major and trace element compositions of garnet amphibolites.

Supplemental Material: Eocene thickening without extra heat in a collisional orogenic belt: A record from Eocene metamorphism in mafic dike swarms within the Tethyan Himalaya, southern Tibet

Table S1: EPMA data of minerals compositions for garnet amphibolite; Table S2: Zircon U-Pb SHRIMP data for garnet amphibolite T0526; Table S3: Major and trace element compositions of garnet amphibolites.

Pre-Variscan granitoids with adakitic signature at west Getic basement of the South Carpathians (Romania): constraints on genesis and timing based on whole-rock and zircon geochemistry

Research on two strata-like intrusions from Slatina-Timiş (STG) and Buchin (BG) at West Getic Domain of the South Carpathians (Semenic Mountains) identified granitoids with adakitic signature in a continental collision environment. Whole-rock geochemical composition with high Na2O, Al2O3 and Sr, depleted Y (18ppm) and HREE (Yb 1.8ppm) contents, high Sr/Y (40), (La/Yb)N (10) ratios and no Eu anomalies overlaps the High-Silica Adakites (HSA) main characteristics, though there are differences related to lower Mg#, heavy metal contents and slightly increased 87Sr/86Sr ratios. Comparison with HSA, Tonalite-Trondhjemite-Granodiorite (TTG) rocks and melts from experiments on basaltic sources suggests partial melting at pressures exceeding 1.25GPa and temperatures of 800-900ºC (confirmed by calculated Ti-in zircon temperatures) as the main genetic process, leaving residues of garnet amphibolite, garnet granulite or eclogite type. The adakitic signature along with geochemical variations observed in the STG-BG rocks indicate oceanic source melts affected by increasing mantle influence and decreasing crustal input that may restrict the tectonic setting to slab melting during a subduction at low angle conditions. An alternative model relates the STG-BG magma genesis to garnet-amphibolite and eclogite partial melting due to decompression and heating at crustal depth of 60-50km during syn-subduction exhumation of eclogitized slab fragments and mantle cumulates. The granitoids were entrained into a buoyant mélange during collision and placed randomly between two continental units. U-Pb zircon ages obtained by LA-ICP-MS and interpreted as Ordovician igneous crystallization time and Variscan recrystallization imprint are confirmed by trace-element characteristics of the dated zircon zones, connecting the STG-BG magmatism to a pre-Variscan subduction-collision event. The rich zircon inheritance reveals Neoproterozoic juvenile source and older crustal components represented by Neoarchean to Paleoproterozoic zircons.

Metamorphism and geochronology of garnet amphibolite from the Beishan Orogen, southern Central Asian Orogenic Belt: Constraints from P-T path and zircon U-Pb dating

<p>Numerous lenses of garnet amphibolite occur in the garnet-bearing biotite-plagioclase gneiss belt in the Baishan area of the Beishan Orogen, which connects the Tianshan Orogen to the west and the Mongolia-Xing’anling Orogen to the east. According to the microstructures, mineral relationships, and geothermobarometry, four stages of mineral assemblages have been identified as follows: (1) a pre-peak stage, which is recorded by the cores of garnet together with core-inclusions of plagioclase (Pl<sub>1</sub>); (2) a peak stage, which is recorded by the mantles of garnet together with mantle-inclusions of plagioclase (Pl<sub>2</sub>) + amphibole (Amp<sub>1</sub>) + Ilmenite (Ilm<sub>1</sub>) + biotite (Bt<sub>1</sub>), developed at temperature-pressure (P-T) conditions of 818.9–836.5 °C and 7.3–9.2 kbar; (3) a retrograde stage, which is recorded by garnet rims + plagioclase (Pl<sub>3</sub>) + amphibole (Amp<sub>2</sub>) + orthopyroxene (Opx<sub>1</sub>) + biotite (Bt<sub>2</sub>) + Ilmenite (Ilm<sub>2</sub>), developed at P-T conditions of 796.1–836.9 °C and 5.6–7.5 kbar; (4) a symplectitic stage, which is recorded by plagioclase (Pl<sub>4</sub>) + orthopyroxene (Opx<sub>2</sub>) + amphibole (Amp<sub>3</sub>) + biotite (Bt<sub>3</sub>) symplectites, developed at P-T conditions of 732 ± 59.6 °C and 6.1 ± 0.6 kbar. Moreover, the U-Pb dating of the Beishan garnet amphibolite indicates an age of 301.9 ± 4.7 Ma for the protolith and 281.4 ± 8.5 Ma for the peak metamorphic age. Therefore, the mineral assemblage, P-T conditions, and zircon U-Pb ages of the Beishan garnet amphibolite define a near-isothermal decompression of a clockwise P-T-t (Pressure-Temperature-time) path, indicating the presence of over thickened continental crust in the Huaniushan arc until the Early Permian, then the southern Beishan area underwent a continental crust tectonic thinning process.</p>

Gore Mountain Garnet Amphibolite records UHT Conditions: Implications for the Rheology of the Lower Continental Crust During Orogenesis

The Gore Mountain Garnet Amphibolite (GMGA), part of the Mesoproterozoic Grenville province in the Adirondack Highlands, NY, USA, is an iconic rock type known for hosting the world’s largest garnets (up to 1 m diameter). We present a new detailed petrographic study of these rocks. Field relations, whole rock, and mineral major and trace element chemistry suggest that these rocks formed via a prograde hydration reaction of a metagabbro during an increase in pressure and temperature. Laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb geochronology applied to zircon interpreted to be metamorphic in origin dates this reaction to 1053.9±5.4 Ma (2σ; MSWD = 0.94), during the Ottawan Orogeny (1090-1020 Ma). Our results on peak metamorphic P-T conditions based on thermobarometry, diffusion models, and thermodynamic modelling indicate that these rocks formed at ultra-high temperature (UHT, >900˚C) conditions (P = 9–10 kbar, T = 950±40˚C), significantly hotter than previously estimated. Diffusion models pinned by nearby cooling ages require the GMGA to initially cool quickly (9.1 ˚C Myr<su-1p>), followed by slower cooling (2.6 ˚C Myr<su-1p>). The two-stage cooling history for the GMGA could reflect initial advection-dominated cooling followed by conduction-dominated cooling once flow ceases. Our results suggest that the region was hot enough to undergo topography-driven lower crustal flow similar to that hypothesized for modern Tibet for 20–0 Myr (25–0 Myr when the effects of melt are included).