Exhumation of high and ultrahigh-pressure rocks in the Qinling-Dabie orogen, eastern China and the Yagan-Onch Hayrhan metamorphic core complex, southern Mongolia.

摘要

The Hong'an block offers a unique opportunity to investigate the rates and mechanisms of exhumation of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks in the Qinling--Dabie orogen in eastern China. Normal-sense shear along the north-dipping Huwan detachment at the northern edge of the Hong'an block occurred between 237--231 Ma. This detachment facilitated the bulk of the exhumation of the HP--UHP rocks, contemporaneous with top-to-south thrusting along the southern margin of the Hong'an block. 40Ar/39Ar ages, combined with U/Pb zircon and Sm/Nd ages, suggest that exhumation of UHP rocks from mantle depths occurred at rates of 5--25 mm/yr from ∼ 245 to 230 Ma. HP--UHP eclogite facies rocks are exposed in a warped extensional footwall, within which kinematic indicators in the eclogite units universally show top-to-north shear. This deformation was accompanied by retrograde metamorphism at amphibolite-to greenschist-facies conditions. Locally, younger northeast-southwest subhorizontal extension is recorded in ductile to brittle fabrics, constrained by white mica. 40Ar/ 39Ar ages between 198--194 Ma. An Early Cretaceous dextral shear zone along the southwest boundary of the orogen was synchronous with plutonism and normal to sinistral-oblique slip along the Xiaotian--Mozitang fault along the northern boundary of the Dabie Shan. Coeval dextral and sinistral shear zones along the northern and southwestern margins of these blocks would have caused eastward lateral extrusion---perhaps driven by collision of the Lhasa block with Eurasia.; Mylonitic rocks associated with the south-dipping detachment fault of the Yagan--Onch Hayrhan metamorphic core complex in southernmost Mongolia indicate subhorizontal south-southeast-directed extension in the Early Cretaceous; synkinematic biotites give 40Ar/39Ar ages of 129 to 126 Ma. The Yagan--Onch Hayrhan core complex demonstrates that late Mesozoic localized high-strain extension, recently recognized in other parts of eastern Asia, also occurred in Mongolia. Mesozoic metamorphic rocks at Onch Hayrhan, previously presumed to be Precambrian, bring into question the existence of the South Gobi microcontinent.