Liu Changshi

The Hercynian-Indonesian collision type granites of west Yunnan and their tectonic significance

Abstract The Linchang Composite Granite Batholith (LCGB) of west Yunnan is an important constituent of the collision-related granite belt of a Permian-Triassic suture zone in this area. It consists of Pinhejie granite in the north and Linchang granite (LG) in the south, which are connected with each other at depth. The Pinhejie granite is N.N.W.-trending, 70 km long, 3–9 km wide and 450 km 2 in area. Its eastern part is Pinhejie gneissic granite (PGG), with an Rb-Sr isotopic age of 309 ± 27 Ma and an initial Sr ratio of 0.7153 ± 3. Its western part is Pinhejie massive granite (PMG), with an Rb-Sr isotopic age of 294 ± 6 Ma and an initial Sr ratio of 0.7205 ± 3. The LG is an extremely large batholith, with a total length of 370 km, a width of 18–36 km and a total area of some 10000 km 2 . The Rb-Sr age of Rb-Sr isochron line for LG and PMG could be obtained with an age of 292 ± 15 Ma and an initial Sr ratio of 0.7216 ± 7. These data show that emplacement of LCGB took place approximately in the range of 292-275 Ma, i.e. in the Permian period. The modelling composition of source materials for LG is equivalent to a mixture consisting of 80% upper crustal Sr plus 20% lower crustal Sr. It suggests that the collision-related LCGB is a typical peraluminous S type granite in genesis.

Geochemical characteristics and genesis of topaz-bearing porphyries in Yangbin area of Taishun County, Zhejiang Province

The host rocks of the porphyry tin deposits in the Yangbin area are principally topaz-bearing porphyry dikes about 2 km long and 2–20m wide. Three lithologie types are identified for the dikes: topaz-bearing potassium feldspar granitic porphyry, topaz-bearing monzonitic granitic porphyry and topaz-bearing quartz porphyry. The content of topaz in the rocks ranges from 10 to 20 vol.%. Porphyritic texture is characteristic, with quartz, potassium feldspar and albite as main phenocryst minerals. The phenocryst occupies 10–20 vol% of the rocks. The rock groundmass consists of subhedral topaz, quartz and protolithionite. Topaz has a unit-cell parameter b=8.797 (Å), and F:OH=1.92:0.18, indicating a F-rich variety formed at high temperature. The topaz-bearing porphyries occurring in this area are strongly peraluminous (A/NKC=1.574–12.94), with high ratios of F/Cl (146–303) and Rb/Sr (5–122). They are rich in incompatible elements (Sn, 313 × 10−6–1042 × 10−6; W, 6 × 10−6–218 × 10−6; Nb, 27 × 10−6–54 × 10−6), but poor in compatible elements (Sr, 10 × 10−6–28 × 10−6; Ba, 58 × 10−6–73 × 10−6; V, 3 × 10−6–12 × 10−6, Cl, 150 × 10−6–226 × 10−6). The rocks are also characterized by high total REE amount (281.69 × 10−6–319.76 × 10−6), with strong Eu depletion (δEu=0.01–0.03) and low ratio of LREE/HREE (0.78–0.84). In summary, the authors propose an idea of S-type genesis for the topaz-bearing porphyries with tin mineralization, instead of I-type.

Relationship between the compositions and unit-cell parameters of biotites from the Taiping-Huangshan batholith in southeast China

The Taiping-Huangshan batholith in southeast China is a polygenetic composite granitoid consisting of two parts. The northern part is known as the Taiping granodiorite, genetically designated to the Syntexis-type granitoids of southeast China[1] or I-type[4]. Based on whole-rock Rb-Sr isochron dating, its age is 22 3 m. y. with a relatively low initial Sr ratio (0.7069), suggesting that it was emplaced during the Indosinian period. The southern part, the Huangshan multi-staged granite, is genetically ascribed to the transformation type of southeast China, or S-type, which includes two bodies known as the Huangshan and Shizilin granites. Both were formed in the late Yenshanian period as indicated by their ages: 131 m.y. for the Huangshan granite and 123 m. y. for the Shizilin granite, and high initial Sr ratios (0.714 for the Huangshan granite and 0.718 for the Shizilin granite) determine using whole-rock Rb-Sr isochron dating method. The biotite in the Taiping granodiorite is Mg-biotite with a high Mg/(Mg +ΣFe + Mn +VIAl3+) ratio (> 0.42) in its octahedral layers, and has a longerc-axis (10.220–10.224 Å) and a larger unit-cell volume (499.5 Å3), as calculated from X-ray powder diffraction data But the biotites in the Huangshan and Shizilin granies occur mostly as siderophyllite, subsequently as Fe-biotite and occasionally as zoned mica consisting of siderophyllite and musco-vite. The Mg/(Mg +ΣFe + Mn +VIAl3+) ratios are low, whileVIAl3+/(Mg+ΣFe+Mn+VIAl3+) ratios high in their octahedral layers. They have a shorterc-axis (10.128–10.196A) and a small nit-cell volume (494.5 Å3 on average). Comparison of their chemical compositions and corresponding unit-cell parameters for 13 samples from the batholith indicates that thec-axis length increases with increasing Mg content with a noticea ble positive correlation between them, but decreases with increasingVAl3+ content, showing an obvious negative correlation. The changes of theb-axis lengths with the compositions of biotites are much less pronounced than that of thec-axis length.

The Tin-Bearing Topaz-Quartz Porphyry, Yangbin Area, Taishun County, Zhejiang Province A Fluid Inclusion Study

The tin-bearing topaz-quartz porphyry in the Yanbin area is subvolcanic in origin, derived from granitic residual magma through strong crystallization differentiation. The rock contains various types of inclusions (hydroxyl-melt, vapor-rich, halite-bearing multiphase, and liquid-rich), which permits us to trace the evolutionary path of the fluid. It is suggested that immiscibility took place in a pulsating manner between melts and fluids during the rock-forming process with the homogenization temperatures ranging from 580°C to 180°C and the salinities varying from low through high to low (5.1–10. 5»6. 9-21.4»30-48»16-22»2-10 in wt% NaCl). The hydroxyl-melt inclusions are considered as evidence of magma-hydrothermal transition.