Four samples from a Permian reservoir in the Ordos Basin of North China were separated into twelve fractions in grain sizes of 〈0.5, 0.5-1 and 1-2 μm. Using the ^40Ar-^39Ar step-heating method, all of the fractions essentially yielded plateau ages ranging from 172.5 to 217.1 Ma. These scattered plateau ages might not have been obtained from pure diagenetic illites but from mixed clay minerals, although the samples were disaggregated using a gentle freeze-thaw cycle to free them of non-clay minerals. A regional thermal event, as suggested by several proxies, led to intensive iHitization as a distinct diagenetic process when the Yanshanian Movement triggered magmatism around the entire North China Block during the Jurassic to Cretaceous. Thermal illites formed during a short time period, whereas detrital illites came from various sources. The scattered plateau ages could have resulted from mixed degassing of thermal and detrital illites. Within one sample, the plateau ages decrease with the diminution of grain sizes, but it is difficult to extrapolate to the detrital-illite-free ages. Because the plateau age is a mixture of ages for thermal and detrital illites, this regression analysis studies the dependence of the plateau ages on the synthetic values of contents and ages of detrital illites instead of on the grain sizes. Comparing the samples to one another, the plateau ages show the same trend among the different grain sizes. Weighted by the contents and ages of detrital illites, linear regression analysis revealed the relationship between the plateau ages and the relative weight proportions. Based on iterated calculations, a thermal event age and a set of weight proportions were derived. The regressed thermal event age is 163.3±1.6 Ma, which coincides with regional thermal activities and links to gas accumulation.
Longzhang WangLiyun WangPing'an PengTongmo DaiDayong Liu
ABSTRACT： The joint methods of ^40Ar/^39Ar laser stepwise heating and in vacuo crushing have been applied to date amphiboles from the North Qaidam ultra-high pressure metamorphic amphibolites. Two amphibole samples analyzed by laser heating yielded saddle-shaped age spectra with total gas ages of 574.5±2.5 and 562.5=±2.5 Ma. These ages are much older than the reported zircon U-Pb ages （-495 Ma） from Yuka eclogite, indicating the presence of excess ^40Ar. In order to decipher the occur- rence of excess ^40Ar and constrain the age of amphibolite-facies retrogression, two duplicate amphibole samples were further employed for ^40Ar/^39Ar in vacuo crushing analyses. Both samples exhibit similar monotonically declining release spectra, which are characterized by rapid decline of anomalously old apparent ages in the early steps. The data of the late steps yielded concordant apparent ages with pla- teau ages of 460.9±1.2 and 459.6±1.8 Ma. We interpret that gases released in the early steps derive from the significant excess ^40Ar containing secondary fluid inclusions （SFIs） due to their distribution characteristics along cracks leading to be easily extracted, whereas those released in the later steps rep- resent the contribution of the small primary fluid inclusions （PFIs）.
Seventeen new ^40Ar/^39Ar analyses reported for ten Tarim Precambrian basement samples from the Kuluketage area, are applied to reconstruct the regional thermo-tectonic history together with previously published data. Eight samples were taken adjacent to the Xingdi and Xinger faults, major structures in the study area, whereas a further two were sampled at some distance from the faults. 4^40Ar/^39Ar data from the latter record rapid cooling following a Neoproterozoic magmatic/metamorphic event and mild Paleozoic thermal disturbance. Paleozoic ^40Ar/^39Ar ages from the study area, as well as from the Central Tianshan and eastern Southern Tianshan suggest two strong deformational periods at ~390 Ma and ~300 Ma. During the older period, argon isotopic systems were reset/disturbed by high temperature related to arc magmatism resulting from subduction of the South Tianshan paleo-oceanic crust, possibly in combination with reactivation of Precambrian faults. The younger period is characterized by widespread late Carboniferous-early Permian intracontinental deformation, which is related to the final amalgamation of the Central Asian Orogenic Belt. Previously published apatite fission track data attest to a long history of post-collisional cooling, which is attributed to continued propagation of deformation within the Central Asian Orogenic Belt.
The Tiegelongnan deposit is a newly discovered super-large porphyry-epithermal Cu-（Au） deposit in the western part of the Bangong Co-Nujiang metallogenic belt, Tibet（China）. Field geology and geochronology indicate that the porphyry mineralization was closely related to the Early Cretaceous intermediate-felsic intrusions（ca. 123–120 Ma）. Various epithermal ore and gangue mineral types were discovered in the middle-shallow part of the orebody, indicating the presence of epithermal mineralization at Tiegelongnan. Potassic, propylitic, phyllic and advanced argillic alteration zones were identified. 40Ar/39Ar dating of hydrothermal biotite（potassic zone）, sericite（phyllic zone）, and alunite（advanced argillic zone） in/around the ore-bearing granodiorite porphyry yielded 121.1±0.6 Ma（1σ）, 120.8±0.7 Ma（1σ） and 117.9±1.6 Ma（1σ）, respectively. Five hydrothermal mineralization stages were identified, of which the Stage IV pyrite was Rb-Sr dated to be 117.5±1.8 Ma（2σ）, representing the end of epithermal mineralization. Field geology and geochronology suggest that both the epithermal and porphyry mineralization belong to the same magmatic-hydrothermal system. The Tiegelongnan super-large Cu-（Au） deposit may have undergone a prolonged magmatichydrothermal evolution, with the major mineralization event occurring at ca.120–117Ma.
LIN BinCHEN YuchuanTANG JuxingWANG QinSONG YangYANG ChaoWANG WenleiHE WenZHANG Lejun
