The SHRIMP U-Pb Dating, Genetic Type And The Tectonic Enviroment Of The Early Paleozoic Ophiolite In East Junggar, Xinjiang

As animportant part of the Central Asian Orogenic Belt(CAOB),the EastJunggar orogenicbelt lies in the middle of the CAOB and is the key area to study the tectonic Evolution of CAOB during the Palaeozoic.The Zhaheba-Aermantai-Beitashan ophiolitite belong to Eastern Junggar orogenic belt,and it formed in the early Palaeozoic. In this dissertation, based on the previous studying results, we detailed study the Zhaheba-Aermantai-Beitashan ophiolite, include dated the ages of ophiolite using the SHRIMP U-Pb dating, measured the zircon Oxygen isotopes and the geochenistry of rocks, discussed the type of genesis and tectonic settings. This study can provide the reliable basis for understanding of tectonic evolution history of the East Junggar.The Zhaheba-Aermantai-Beitashan ophiolitic belt was strongly fractured due to the intense tectonic activities, and the outcrops in the Zhaheba, Aermantai and Beitashan areas along the fracturing zone. The outcropping area and lithology in each area is different with each other, but the rock units are relatively completed as a whole, (1) ultramafic rocks:harzburgiteand and dunite; (2) mafic cumulates:websterite and gabbro; (3) leucocratic rocks:trondhjemite, albitite and albite granites; (4) volcanic lavas: basalts.The gabrros in the Zhahebaophiolite belt yields the SHRIMP U-Pb ages of 488±2 Ma (n=19; χ2=1.05) and 481±4 Ma (n=9,χ2=0.89),with theweighted mean δ18O value of 5.0 ±0.2‰ (1σ) and 5.1 ±0.3‰(1σ),consistent with the mantle value of 5.3±0.3‰, and therefore we interpret the age of 488± 2 Maand 481±4 Ma as the formation age of the gabbro. The trondhjemite yields the ages of 485±3Ma (n=11,χ2=0.98), weighted mean δ18O value of 5.2±0.2‰ (1σ), consistent with the mantle value of 5.3±0.3‰, so it represents the formation age of the trondhjemite, and further determined the formative age of the Zhaheba ophiolite, namely 488-481 Ma. Moreover, the geochemistry of corresponding the rock units indicate that the ophiolite belt was influenced by the initial subduction during in the forming time of the ophiolite.The gabrrosin the Aermantai ophiolite belt yield the zircon SHRIMP U-Pb ages of 506±2 Ma (n=16,χ2=0.94)、515±3 Ma (n=17,χ2=0.30) and 516±4 Ma (n=17,χ2 =0.07).with the weighted mean δ18O value of 5.5±0.1‰(1σ),5.1±0.1‰(1σ) and 5.2 ± 0.2%o(la), respectively,onsistent with the mantle value of 5.3±0.3%o. Therefore, it represents the crystallzing age of the gabrro, and it almost determined the formative age of the Aermantai ophiolite. Based on the geochemistry of the gabrro, the REE in the diagrams shows the relative flat pattern, with no distinct differentiation. No Nb-Tan egative anomalies are identified in the trace element diagrams, and these characteristics indicate that the gabrro in the Aermantai ophiolite was slightly or not affected by subduction activities.The gabrro from the Beitashan ophiolite yield SHRIMP U-Pb ages of 494 ±3 Ma (χ2=1.16)、495± Ma (n=20,χ2= 0-07) and 497±3 Ma(n=15,χ2= 0.37), with the weighted mean δ18O value of5.2±0.3‰ (1σ),5.2±0.3‰ (1σ) and 5.2±0.3‰ (1σ) and they are very close to the Oxygen isotopes of mantle zircon (δ18Omantle=5.3±0.3‰), indicate that those are crystallizing ages og grabbros, and further represent the formative age of the Beitashan ophiolite. The normalized REE diagram of gabrro shows the pattern of depletion in LREE and enrichment in HREE.and shows the signature of enrichment in LILE and with certain negative anomaly in Nb. Those chemical features indicate that the gabrro are affected by subduction during the formation.A granite porphyry, which intrudes ophiolite belt, yielded a weighted mean age of 407 ±2 Ma and a weighted mean δ18O value of 6.1±0.2%o (1σ). We interpret the age as the formation time of the porphyry and the δ18O value as an indicator for the granitic melt originated from partial melting of basaltic oceanic crustal materials. The formation time of the porphyry constrained the closure of Aermantai ocean basin in the late Silurian-early Devonian. Meantime, the grabbros in Beitashan and Zhaheba ophiolite have a younger zircon U-Pb age and the age is close to that of the granite porphyry. Combined with the previous research and the geochemistry from the dissertation, it’s possible that they formed during the closure of the Zhaheba-Aermantai-Beitashan oceanic basin, which provide us the new constraint on the closure f oceanic basin.Based on the previous studies, the author found that one type of basalt hosted in the Aermantai ophiolite shows the signature of SSZ type of ophiolite, and it shows us its geochemical characteristics as:(1) N-MORB-like REE pattern with depletion in LREE. (2) slightly lower ratio of Ti/V relative to N-MORB,(3) enrichment in mobile elements and LILEs, (4) No or weak deep chemical signatures. All of the features indicate that they belong to the fore-arc basalt, and the signature rock of the SSZ-type ophiolite. These features indicated thatZhaheba-Aermantai-Beitashanophiolitebelt maybe formed in the the fore-arc environment, and further reflect that the time of initial subduction.Meanwhile, the arc basalts that outcropped in the Zhaheba-Aermantai-Beitashan ophiolite belt,and geochemistry indicate the deep subduction signal. Those also direct to the construction of subduction-mantle system, and the deep components influx into the arc magmatism. This type of volcanic rock indicates that the initial subduction has switched into real intense subduction.