Bangong Lake nickel-sulfur causes of ultrabasic rocks and the process of crustal extension

Title: Bangong Lake nickel-sulfur causes of ultrabasic rocks and the process of crustal extension Author: Jiang Junhua Degree awarded: Chinese Academy of Geological Sciences Key words: nickel-containing ultramafic rock;; dykes;; mantle;; crustal extension;; Bangong island arc belt;; the Tibetan Plateau Summary:

Qinghai-Tibet Plateau Bangong - Nu ore belt of ultramafic rocks of nickel sulfide ore was discovered in Tibet in recent years, new types of mineralization. Based on the ore-forming regions with sulfur western Bangong nickel ultrabasic rocks (hereinafter referred to as nickel-containing ultramafic rock) rock geochemistry and physical Sr, Pb Magnetic lifter isotopic analysis, discusses the nickel-containing ultramafic magma the source region and the formation conditions, and in accordance with zircon U-Pb LAICPMS dating results, northwest of the base of Tibetan background. The region of acid dykes study shows that these ancient basement material recycling process may be involved in the formation of these dikes of magmatic activity.

The study found that the region Bangong nickel ultramafic rock geochemistry to enriched in large ion incompatible elements U, Sr, Pb; loss of high field strength elements Nb, Ti, enrichment of Ta. Low total rare earth elements, standardization curves intersect, light and heavy rare earth fractionation of low performance of flat-type, δEu from weak negative - positive anomaly in - heavy rare earth elements were flat or slightly concave to the distribution patterns, Description magma should have residual amphibole. Trace elements and rare earth elements show the same increase in the ore-bearing magma in the process of assimilation of crustal material received on the contamination is determined by the type of turbidite and the Indian Ocean MORB-type mantle mixed results. Shallow depth of magma generation, for the spinel lherzolite facies, partial melting of the source region roughly 10% -15%.

Rocks in the emergence of a 2.479 billion-year focus on the residual zircon age, marking time in the Waters Bangong ocean basin sediments are more single source area, mainly from the late Archean - early Proterozoic basement of the old, and presumably at the turn of Archean and Proterozoic (2.5 billion years) may be an ancient Tibetan crust northwest of rapid growth period.

Bangong regional north-south and east-west veins into granite rock types and diorite porphyry, system analysis and geochemical zircon LA-ICP-MS U-Pb dating of that class of these dikes represent Tethys public lake basin crustal extension after the incident closed. Which only north-south to the output of the granite porphyry dikes formed at (79.59 ± 0.32) Ma (MSWD = 1.08); both north to south, there are things to output diorite formed (75.9 ± 1.2) Ma (MSWD = 2.8). Two kinds of veins are enriched in large ion incompatible elements Rb, U, Th, K, Pb http://www.999magnet.com/products/131-magnetic-lifter and depleted in high field strength elements Nb, Ti. Rich granite porphyry which SiO2, K2O, poor Na2O, CaO, Fe2O3 (T), Sr, Ti strongly depleted, Nb / Ta ratio is low, the strong negative Eu anomalies, indicate their magmatic source for the amphibolite facies, the formation of shallow depth. The diorite-Si02 low levels, K2O, Na2O, CaO content and the granite porphyry rather, Fe2O3 (T) levels increased, geochemistry, Sr showed a weak positive anomaly, Nb / Ta ratio of high-, Ti weak losses, negative weak Eu anomaly, indicating that their magma source is likely to eclogite or garnet amphibolite facies. Sr, Nd isotopic characteristics indicate two dykes and Gangdese orogenic belt adakitic porphyry copper similar, both from the lower crust, and Pb isotopes indicates that their source FOZO end-member, three combined tracer that its source composition containing both crust and mantle, combined with Nd depleted mantle age characteristics, the authors speculated that these veins are Neoproterozoic igneous rocks from the FOZO and EM Ⅱ separate end-underplating of basaltic rocks under the crust after the result of partial melting. Degree Year: 2010