C. A. Menold

Early Paleozoic Tectonic and Thermomechanical Evolution of Ultrahigh-Pressure (UHP) Metamorphic Rocks in the Northern Tibetan Plateau, Northwest China

Coesite- and diamond-bearing ultrahigh-pressure (UHP) metamorphic rocks represent continental materials that were once subducted to depths of >90 km. Identifying how these rocks were subsequently returned to Earths surface has been a major challenge. Opinions on this matter vary widely, ranging from vertical extrusion of a coherent continental slab to channel flow of tectonically mixed mélange. To address this problem, we conducted integrated research across the North Qaidam UHP metamorphic belt using structural mapping, petrologic studies, and geochronologic and thermochronologic analyses. Our regional synthesis indicates that the early Paleozoic Qilian orogen, within which the North Qaidam UHP metamorphic belt was developed, was created by protracted southward oceanic subduction. The process produced a wide mélange belt and the Qilian magmatic arc. Arc magmatism was active between 520 and 400 Ma, coeval with North Qaidam UHP metamorphism. The North Qaidam UHP metamorphic belt also spatially overlaps the early Paleozoic Qilian magmatic arc. Petrologic, geochronologic, and geochemical studies indicate that the protolith of the UHP metamorphic rocks was a mixture of continental and mafic/ultramafic materials, derived either from oceanic mélanges or pieces of a rifted continental margin tectonically incorporated into an oceanic subduction channel. These observations require that the North Qaidam UHP metamorphic rocks originated at least in part from continental crust that was subducted to mantle depths and then transported across a mantle wedge into a coeval arc during oceanic subduction. Upward transport of the UHP rocks may have been accommodated by rising diapirs launched from a mélange channel on top of an oceanic subducting slab. To test this hypothesis, we developed a quantitative model that incorporates existing knowledge on thermal structures of subduction zones into the mechanics of diapir transport. Using this model, we are able to track P-T and T-t paths of individual diapirs and compare them with the observed P-T and T-t paths from North Qaidam. The main physical insight gained from our modeling is that the large variation of the observed North Qaidam P-T paths can be explained by a combination of temporal and spatial variation of thermal structure and mechanical strength of the mantle wedge above the early Paleozoic Qilian subduction slab. Hotter P-T trajectories can be explained by a high initial temperature (~800°C) of a diapir that travels across a relatively strong mantle wedge (i.e., activation energy E = 350 kJ/mol for dry olivine), while cooler P-T paths may be explained by a diapir with initially low temperature (~700°C) that traveled through a weaker mantle wedge, with its strength at least two orders of magnitude lower than that of dry olivine. This latter condition could have been achieved by hydraulic weakening of olivine aggregates in the mantle wedge via fluid percolation through the mantle wedge during oceanic subduction.

High- and Ultrahigh-Pressure Metamorphism in the North Qaidam and South Altyn Terranes, Western China

The North Qaidam and South Altyn terranes extend approximately 1000 km across the northern Tibetan Plateau, and five localities preserve evidence of Early Paleozoic high-pressure (HP) or ultrahigh-pressure (UHP) metamorphism, including the presence of coesite, coesite pseudomorphs, and diamond. A review of the geology, petrology, and geochronology collected over the past 10 years since these localities were discovered supports a correlation of the North Qaidam and South Altyn terranes, offset 350-400 km across the Altyn Tagh fault. Geochronology interpreted to reflect eclogite-facies metamorphism yields ages between 500 and 420 Ma; detailed geochronology from one locality supports a protracted (tens of m.y.) history of HP/UHP metamorphism. Rock associations and geochronology support a passive-margin origin for the protolith of the HP/UHP rocks, which received sediments from a Proterozoic-Late Archean source, and was intruded by Neoproterozoic granites derived from crustal melting.

Petrochronology of Himalayan ultrahigh-pressure eclogite

The timing and nature of the India-Asia collision, Earth’s largest ongoing continent-continent collisional orogen, are unclear. Ultrahigh-pressure metamorphism of Indian continental margin rocks is used as a proxy for initial collision because it indicates subduction of India. Records of this metamorphism are preserved only at Kaghan Valley (Pakistan) and Tso Morari (Ladakh, India), separated by ~500 km and having published ages of peak pressure of 46.2 ± 0.7 Ma and 53–51 Ma, respectively. The apparent ~6 m.y. age difference may refl ect multiple subduction events, a large promontory along the former Indian margin, or inadequate constraints on the time of peak pressure recrystallization at Tso Morari. We present 108 coupled, in situ U/Th-Pb and rare earth element (REE) analyses of zircons in two Tso Morari eclogites to obtain age and petrologic information. The ages range from ca. 53 Ma to 37 Ma, and peak at ca. 47–43 Ma. Flat heavy REE slopes and the absence of an Eu anomaly are compatible with eclogite-facies zircon (re)crystallization. This (re)crystallization probably occurred at ultrahigh pressure, because 64% of the analyses are from zircon included in ultrahigh-pressure garnet and omphacite. These results are consistent with those from Kaghan Valley, and suggest that a single, protracted ultrahigh-pressure metamorphic event occurred contemporaneously across much of the orogen, following initial contact of the Indian and Asian continents at ca. 51 Ma or later.

White mica trace element and boron isotope evidence for distinctive infiltration events during exhumation of deeply subducted continental crust

ABSTRACT Previous study of subducted continental crust within the Luliang Shan terrane in Northwest China has documented metasomatic formation of thick, hydrated phengite + garnet-rich selvages at the interface between mafic eclogite blocks and quartzofeldspathic host gneiss. Whole rock concentrations of Cs and Ba within the selvage are enriched by two orders of magnitude relative to the eclogite blocks and host gneiss. We performed in situ ion microprobe analyses of Li, Be, B, Rb, Sr, Cs and Ba and δ11B of phengite within the Luliang Shane terrane to better constrain the source(s) of the infiltrating fluid. The phengite within the selvage are enriched in Li, Cs and Ba and yield δ11B values between −30‰ and −9‰, values that are lower than mantle values. High Ba/Rb, Cs/Rb coupled with low B/Be, B/Li and highly negative δ11B values indicate that the high-pressure fluid that formed the selvage was derived from highly devolatilized rocks within the subduction channel. In contrast, muscovite, which crystallized in the adjacent host gneiss during a subsequent lower pressure phase of fluid infiltration at approximately 0.9 GPa depths, has much lower Li, Cs and Ba relative to the high-pressure phengite. These retrograde muscovite have very high concentrations of B (up to 5500 ppm) and Be (up to 50 ppm) and high (−2 to +8‰) δ11B values that are consistent with crystallization from a fluid derived from shallower and less devolatilized regions of the subduction zone. Additional host gneiss samples, regionally distributed and kilometres away from the studied area lack the B-rich signature and indicate that the late stage fluids were likely localized to the region near the studied traverse.