Discovery of especially large bubble-like miarolitic granites in Laoshan and its geological and geomorphic significance
Recently, some especially large bubble-like miarolitic caves in A-type granites in Laoshan have been discovered with the largest diameter of up to 3.1 m, far exceeding the granitic miarolitic caves previously reported, thus adding new knowledge to geology. This helps to study the primary environment for granites formed in Laoshan and also reveal genesis of “natural-mortar” micro-landform for granites in Laoshan. The existence of a new genesis type—“bubble-like miarolitic caves” of speleology is also confirmed by this discovery. Especially large bubble-like miarolitic caves are rare geological relics with high ornamental, science and tourism value.
The thermal history of the miarolitic granite at Xincun, Fujian Province, China
The thermal history of the late Mesozoic miarolitic granite has been studied based on zircon U-Pb dating, whole rock Rb-Sr dating and K-Ar dating of muscovite, biotite and K-feldspar from the same rock sample. From the beginning of zircon crystallization to the closure of K-Ar system of biotite, the granite body had a slow cooling rate (11.0°C/Ma) and an ascending rate (0.07 mm/a). From the end of this stage to the closure of K-Ar system of K-feldspar, the granite body increased its cooling rate (45°C/Ma) and ascending rate (0.36 mm/a). The thermal history of the Xincun granite with a slow cooling rate at the early stage and a fast cooling rate at the late stage may have been related to the fact that the Fujian coastal area had very high geothermal gradient in the late Mesozoic and evident decrease in geothermal gradient in the early Cenozoic.
RbSr microchrons in the Manaslu granite: implications for Himalayan thermochronology
Abstract Samples of minerals from the Manaslu leucogranite in the central Himalaya were microsampled and measured for RbSr mineral ages. The ages obtained are used in conjunction with models for RbSr closure temperatures, to reconstruct the thermochronological history of the Manaslu granite. The RbSr results indicate a rapid change in temperature between 21 Ma and 17 Ma of 60–70°C/million yr, which, in turn, indicates unroofing at 2 mm/yr. The temperature-time curve inferred from RbSr data is in excellent agreement with existing ArAr data on the Manaslu granite and its thermal aureole. The results confirm that parts of the Himalaya, in addition to the Gangdese belt, were eroded rapidly in the 20-17 Ma time period, and this erosion could account for the observed rapid increase in seawater 87 Sr 86 Sr . A tourmaline-whole rock age of 29.5 Ma was obtained on a sample from the Chokkang Arm, but may be unreliable because of post-crystallization alteration of feldspar. Microsampling approaches are demonstrated that allow high precision isotopic data to be obtained on minerals sampled at scales of 100 μm or less. Microanalysis, combined with models for Sr redistribution, can be a useful thermochronological tool that can augment the 40Ar39Ar approach.