Two alkaline syenite plutons, the Tieshan and Yangfang plutons, have recently been recognized within NE-trending fault zones in eastern Cathaysia, South China. The rocks are very enriched in K2O (6.28-9.39wt.%), rare earth elements (REE; particularly light REE) and large ion lithophile elements, but are relatively low in high field strength elements. Isotopically, they are characterized by high initial 87Sr/86Sr (0.7093 to 0.7123) and low eNd(t) values (5.64 to 10.63). The geochemical data suggest that the alkaline syenites most likely formed via fractional crystallization of enriched mantle-derived magmas. Sensitive High-Resolution Ion Microprobe zircon U-Pb dating indicates that these two intrusions have Late Permian (254F4Ma) and Early Triassic (242F4Ma) crystallization ages, respectively. Our data suggest that a tectonic regime dominated by transtension probably existed from at least the latest Permian into the Triassic and was responsible for the formation of the Tieshan and Yangfang alkaline syenites. When combined with previous paleomagnetic, structural, and sedimentology data, we suggest that the transtension along the NE-trending strike-slip fault zones was related to oblique subduction of the Pacific plate underneath South China.

One hundred and twenty-five mineral grains from 45 visually pure K-bearing Mn oxide (hollandite group) samples collected from weathering profiles in the Mt Tabor region of central Queensland, Australia, were analysed by the 40Ar/39Ar laser probe technique. These K-Mn oxides precipitated mainly through a process of cavity filling (direct precipitation from weathering solution), with botryoidal texture formed by micrometric mineral bands. Well-defined and reproducible plateau ages have been obtained for most samples, ranging from 27.2

采用40Ar/39Ar激光阶段加热分析技术，对采自澳大利亚昆士兰中部某风化剖面中的钡硬锰矿和隐钾锰矿进行同位素年龄测定，获得可靠的坪年龄和等时线年龄，再次证明可以用40Ar/39Ar测年技术对极细小的表生锰氧化物进行年代学研究。发现Ar同位素在钡硬锰矿和隐钾锰矿中主要赋存于两种不同的位置。第1种位置（晶间空隙）主要赋存大气成因Ar，在激光功率为0.2～0.4W时释放出来，包括40Aratm, 38Aratmt和Aratm.第2种位置（即隧道结构或晶内位置）主要赋存放射性成因Ar（40Ar*）和核反应成因Ar (39Ark,38Ark)，其去气的激光功率主要在0.5～1.0W之间。当激光功率大于1.0W时，已很少有Ar气从锰氧化物中释放出来，说明赋存放射性成因和核反应成因Ar的隧道结构因持续受热而崩塌、解体。40Ar*,39Ark和38Ark的近于完全一致的去气行为意味着它们不仅在锰氧化物中占有相同或相当的晶体位置，同时证明39Ark在样品接受同位素分析之前的中子照射及实验室预热过程中没有反冲出矿物晶体结构。结果表明，尽管表生锰氧化物中含有大理的大气Ar,且这些氧化物的晶体十分细小，但应用40Ar/39Ar激光阶段加热分析技术可以获得有意义的风化年代学信息。