U-Pb and Ar-40-Ar-39 geochronology of the Baiyunshan gneiss 481 (central Guangdong, south China): constraints on the timing of early Palaeozoic and Mesozoic tectonothermal events in the Wuyun (Wuyi-Yunkai) Orogen

摘要

Composite Sensitive High Resolution Ion Microprobe (SHRIMP) U-Pb zircon and Ar-40-Ar-39 step-heating biotite-hornblende ages are used to provide constraints on the timing and origin of the felsic gneissic rocks in the Baiyunshan Mountains region and to elucidate their tectonothermal history. SHRIMP dating and CL imaging of zircons give magmatic zircon crystallization ages between Late Ordovician and Early Silurian (c. 453.5 Ma, 446 Ma, and 439 Ma) for three representative felsic gneisses, suggesting that most of the Baiyunshan gneiss cannot represent basement rocks of the Cathaysia Block as previously thought. Including the present age information, a synthesis of available age data for regional Wuyun (Wuyi-Yunkai) events reflects the emergence of orogen-wide magmatism that could be syn-orogenic and have occurred mainly between 460 and 420 Ma in the South China Block. Inherited zircons are abundant, with ages clustering at late Mesoproterozoic (1189-1017 Ma) and middle Mesoproterozoic (772 Ma), which reveals that the Baiyunshan orthogneiss samples a crustal basement containing significant igneous or recycled components related to the Rodinia amalgamation and break-up. A SHRIMP date of 212 +/- 12 Ma from a white rim of zircon provides evidence for metamorphic overprinting of an Indosinian tectonothermal event on the Baiyunshan gneiss. Incremental heating experiments with six biotite samples and one hornblende sample from a variety of metamorphic rocks yielded two distinct Ar-40-Ar-39 age groups: 150-155 and 94-98 Ma. The older ages are similar to zircon U-Pb dates for widespread granitic intrusions in central Guangdong. We attribute them to Late Jurassic magmatism-induced thermal resetting of the biotite K-Ar system. On the other hand, the younger age group is interpreted to record either cooling through the biotite closure temperature of similar to 300-350 degrees C or a second resetting of biotite Ar isotopes at c. 94-98 Ma due to contemporaneous magmatic activity. Our present age data suggest that the Maofengshan orthogneiss was exhumed to 8 to 10 km crustal levels at c. 150 Ma, whereas the eastward components of gneissic rock masses appear to have passed upward through the same crustal depth synchronously or later (by c. 94 Ma). Exhumation of middle crustal-level rocks in the study area since c. 155 Ma is roughly coeval with exhumation of gneissic rocks from elsewhere in the Wuyun Orogen, suggesting a large-scale mechanism for the exhumation pulse related to the Yanshanian extensional tectonic regime.