Han Shan Northern Margin Of The Yangtze Gabbro Rock Petrology, Geochemistry And Zircon Age Studies

The Neoproterozoic magmatism of the northwestern margin of the Yangtze block are fierce and strong, and mafic-acidic intrusive complex are developed extensively. These intrusive complexes are considered to be the residual evidence of the plate subducting because of the Neoproterozoic Long Lived seafloor spreading (during950Ma to730Ma) occured in the northwestern margin of the Yangtze block. The period of expansion and subduction results in rifting magmatism happened in back-arc basin along the Jiangnan fold belt of the South China Craton. In the northern margin of Yangtze block, this period of magmatic activities are represented by Hannan intrusive complex that widely develope in Hannan region. So far, many studies have shown that there are enormously different point of views and various genetic pattern involving in the aspect of judgment tectonic setting of Neoproterozoic magmatic activities in Hannan region, such as the view of Neoproterozoic supercontinent breakup and the view of island arc etc. This study mainly acquires the following progresses and conclusions by means of detail geological research and systemic research on petrography, mineralogy, geochemistry, zircon U-Pb geochronology and Hf isotope to analyze Da hanshan garbbros in Hannan intrusive complex.1, Detailed geological studies show that Da hanshan garbbros are located in southwest of Hannan intrusive complex and in centre of the Hanzhong Basin, the whole mountain seems as a huge stook with flat top which founded planation surface, this unique geological features suggest that the rock mass may experience significantly uplift after the formation(relativing to the foothills of the altitude, it exposes more than900M). In addition, the overlying strata and surrounding rocks are higly denuded. The garbbo mass are very fresh and have no significant deformation and metamorphism, indicating that it has been in a state of relatively stable craton after the mass formed (-800Ma).2, Petrographic studies show that the core of the Da hanshan rock mass is considered as hornblende-hypersthene gabbros, the edge of it consists of hornblende gabbros. Cumulus structure can be discovered occasionally on the outcrop. The noritegabbros, grayish black of color and medium to coarse grained massive structure. They are mainly composed of hypersthene (6%-7%), augite (3%-4%), brown and green hornblende (30%), and plagioclase (55%), as well as containing a small amount of zircon, magnetite and apatite. Hornblende gabbros are also gray dark, but with fine-grained massive structure. They are mainly composed of hornblende and plagioclase, including a few residual monoclinic pyroxene; the accessory minerals include zircon, apatite, while the altered minerals are epidotes.For hornblende noritegabbros, mineral chemistry studies reveal that the plagioclase belongs to bytownite and labradorite, monoclinic pyroxene belongs to augite, and orthopyroxene belongs to hypersthene. The brown hornblende which crystallizes in early time and the green hornblende which crystallizes in late time all belong to common hornblende. The crystallization temperature of hornblende noritegabbros is837-944℃by utilizing the Opx-Cpx thermometer.3, Here we collect four pieces of samples, which gather from different positions of the Da hanshan, the zircon of the samples own the features of magmatic Oscillatory zones and fan-shaped zones without core-mantle boundary. The ratio of Th and U ranges from0.46to1.19, LA-ICPMS zircon U-Pb isotope dating results indicates that the206Pb/238U weighted average age are782.7±6.9Ma,779.0±5.9Ma,791.0±4.0Ma and793.0±4.4Ma respectively, the mean age are786.4±5.3Ma, which represents the formation age of Da hanshan gabbros (formed at neoproterozoic).4, The initial ratio of176Hf/177Hf in zircon ranges from0.282504to0.282594, the value of εHf(t) ranges from+6.51～+10.70. This feature indicates that the magma derived from depleted mantle source region.5, Geochemical studies reveal that Da hanshan gabbros present the characteristics of tholeiite, with high FeO(8.61-12.89%), medium MgO(6.69-9.05%), low K2O(0.22-0.57) and Na2O (1.90%～2.42%), the contents of TiO2(0.63%～1.15%) is lower than the cumulate gabbro of oceanic ridge(1.12%) but higher than that of ophiolite gabbro of orogenic belt(0.5%). SREE of a low level range from35X10’6to68×10-6, The REE patterns display slight enrichment in LREE and slight depletion in HREE relativing to LREE. All of the REE patterns and MORB-normalized patterns of Da hanshan gabbros present a slight right dipping tend, i.e., enriched in LREE and delpeted in HREE. It indicates that these gabbros fractionated significantly in the evolution of magma. Comparing with the the cumulate gabbro of oceanic ridge, Da hanshan gabbros are enriched in LREE and delpeted in HREE; nevertheless, they correspond to the feature of immature intra-island arc excepting a slight enrichment in LREE. The spider diagram with enrichment in LILE and slight Nb and Ta depletion suggest that these gabbros present the feature of intra-island arc gabbros. The gabbros may derived from mantle source at the early immature island arc stage after the subduction of oceanic ridge by the evidence of that they display a transitional feature from mid-oceanic tholeiite to island arc tholeiite.6, Integrated the results of petrology, mineral and rock geochemical studies, It can be proposed preliminarily that Da hanshan gabbros are formed at the early immature intra-island arc stage by subduction of oceanic ridge. The zircon U-Pb dating indicates that it has no relationship with Grenvillian orogeny and island arc system indicates that it has no relationship with the breakup of Rodinia.