Massive granitoid production without massive continental-crust growth in the Chinese Altay: Insight into the source rock of granitoids using integrated zircon U-Pb age, Hf-Nd-Sr isotopes and geochemistry

Abstract

This study integrates new and extant zircon U-Pb age and Hf isotope data, whole-rock Nd-Sr isotope data, and geochemistry data from the Chinese Altay (CA) and adjacent areas, in an attempt to identify the source rocks of granitoids and acidic-intermediate volcanic rocks and, hence, continental-crust growth in the CA. The protolith of the gneisses includes the earliest products of erosion from the surface rocks of the island arc volcanics and the syn- and post-collisional granitoids of the Caledonian orogenic-mountain system, whereas the provenance of metasedimentary rocks was exposed from increasingly deeper levels or from an expanded region that evolved from the axis to the flank of the mountain system as it was denuded until planation such that eroded material from the Tuva-Mongol microcontinental block to the east could be delivered to the CA. The CA tends to be more mafic and isotopically more primitive with depth, because material eroded early constitutes the lower section and material eroded late constitutes the upper section of the Early-Paleozoic sedimentary-pile.

Source rocks for the granitoids and acidic-intermediate volcanic rocks of the CA include the eroded material from the microcontinental block, the eroded material from the syn- and post-collisional granitoids of the Caledonian province, MORB-type mafic rocks, and a minor component from the metasomatized lithospheric-mantle. Eroded material from the terrains north of the CA was first deposited as an Early-Paleozoic sedimentary-pile in the CA. The top section of this pile was again eroded and transported to the trench at the CA continental margin, and these second-round sediments were subducted beneath the CA accretionary wedge. The un-eroded lower section of the Early-Paleozoic sedimentary-pile lay at bottom of the CA accretionary wedge that was underplated by MORB-type mafic rocks.

The CA granitoids can be divided into orogenic (460-360 Ma) and post-orogenic (∼320-260 Ma). The orogenic granitoids can be subdivided into Groups 1 (ɛ_Nd (t) < +1) and 2 (ɛ_Nd (t) mostly higher than +1). The main branch of the granitoids (67 data) that includes Group 1 and a majority of Group 2 were derived mainly from eroded materials from the Caledonian province and the Tuva-Mongol microcontinental block. The source rock of acidic-intermediate volcanic rocks was dominantly eroded materials from the latter. The granitoids in the Erqisi nappe at the junction between the CA and the Junggar are characterized by a mixed source between MORB-like mafic rocks and juvenile crustal-rocks. The Mid-Devonian dacite-rhyolite and dolerite dikes in the west Chinese Altay were derived from metasomatized lithospheric-mantle.

Integrated isotope and geochemistry data further constrain the source compositions of granitoids. The granitoids in the northwest Chinese Altay were mainly derived from subducted sediment that was eroded from the top section of the Early-Paleozoic sedimentary-pile, whereas those in the east Chinese Altay were derived dominantly from the lower section of the pile. The source rock of the granitoids in the northeast of the middle Chinese Altay consists of the lower section of the Early-Paleozoic sedimentary-pile; and sediment, quartz keratophyre, and keratophyre that were subducted or thrust beneath the northern Chinese Altay. The main source rock of the granitoids in the southwest of the middle Chinese Altay varies between the subducted sediment in the westernmost segment and the lower section of the Early-Paleozoic sedimentary-pile and the mafic underplate to the east.

A geophysical-sounding profile across the CA and East Junggar reveals a three-layer structure of the CA accretionary-wedge underplated by an oceanic lithosphere that was detached from the downgoing oceanic-spreading-center. From the Silurian to the Mid-Late Devonian, the active spreading-center was repeatedly subducted beneath the CA along a southwestward-retreating subduction zone as represented successively by four NE-dipping thrust faults that converge to a low-angle décollement at depth. Asthenospheric upwelling heated and melted the subducted sediment and the lower section of the Early-Paleozoic sedimentary-pile, along with the mafic underplate and generated the acidic-intermediate volcanic rocks and granitoids that culminated at ∼400 Ma. During the Mid-Devonian, shallow dipping dècollement-like subduction changed to a steeply dipping subduction beneath the lithospheric-mantle wedge. The mantle wedge shielded the CA crust from heating from the subducting spreading-center, which resulted in waning granitoid magmatism from ∼400 Ma to ∼360 Ma.

In conclusion, there was massive granitoid production but not massive continental-crust growth in the CA at ∼400 Ma.