Mid-crustal emplacement and deformation of plutons in an Andean-style continental arc along the northern margin of the North China Block and tectonic implications

摘要

Outcrop mapping and thermobarometrical, thermochronological and microstructural analyses in five Late Carboniferous-Early Permian arc plutons in the northern North China Block (NCB) indicate a transition of tectonic regime from arc-perpendicular contraction to transpression and successive uplift and exhumation of the crustal rocks during magma emplacement and construction of the continental arc system. The early emplaced Daguangding and Boluonuo plutons (324-302Ma) display well-developed, penetrative, moderately to steeply dipping magmatic foliations and high-temperature sub-solidus foliations that are parallel to host rock foliation near the contact. Widespread high-temperature sub-solidus foliations in these plutons indicate that they are syn-tectonic with N-S, arc-perpendicular shortening and record arc construction at a convergent continental margin. However, the late emplaced Tianqiao and Xianghuangqi-Wudaoyingzi plutons (288-274Ma) display relatively weak magmatic fabrics at their margins and lack sub-solidus foliations and pluton-related ductile deformation of the contact host rocks. These structural differences likely reflect a kinematic transition from early arc-perpendicular contraction to late sinistral transpression during continental arc formation. Aluminum-in-hornblende barometry shows that their estimated emplacement depths decrease gradually from the Late Carboniferous (18.7±0.8km) to Early Permian (13.8±0.9km), indicating successive exhumation of the crustal rocks at an average rate of ca. 123mm/ka over 40millionyears of arc formation. ~(40)Ar/~(39)Ar thermochronological results show that cooling of the continental crust was very slow (4.9°C/Ma) during arc construction in the Late Carboniferous-Late Permian but increased significantly during the Late Permian-Middle Triassic after arc termination. The mid-crust of the northern NCB remained very hot (>530°C) during arc construction and emplacement of the arc batholiths. Combined with emplacement depth estimated by aluminum-in-hornblende barometry, paleogeothermal gradients of 42.8-46.5°C/km were estimated in mid- to upper-crustal levels (0-19km). Hot mid-crust and high mid-upper crust geothermal gradients resulted from magmatic advection are likely the most important reasons for crust thickening and uplift during construction of Andean-style continental arcs and largely influenced the rheological behavior of continental crust.