Geochronology and Hf Isotope Characteristics of the Paleozoic Granite in Nalati Area, West Tianshan Mountains
徐学义(Xue-Yi Xu)；王洪亮(Hong-Liang Wang)；马国林(Guo-Lin Ma)；李平(Ping Li)；陈隽璐(Jun-Lu Chen)；李婷(Ting Li)
花岗岩成因 ； 锆石Hf同位素 ； LA-ICPMS锆石U-Pb定年 ； 古生代 ； 西天山那拉提 ； petrogenesis of granite ； zircon Hf isotope ； LA-ICPMS zircon U-Pb dating ； Paleozoic ； Nalati area in West Tianshan Mountains
|Volume or Term/Year and Month of Publication||
29卷6期（2010 / 11 / 25）
691 - 706
西天山那拉提地区发育强烈的古生代花岗岩浆活动，依据野外地质特征和形成时代，将该区花岗岩分为早古生代－晚泥盆世花岗岩和石炭纪花岗岩两类。前者变形较强，发育弱的片麻理构造，岩石类型主体为闪长岩－石英闪长岩－花岗闪长岩－二长花岗岩，LA-ICPMS锆石U-Pb定年揭示其形成时代为366~485 Ma，εHf (t)和t(下标 2DM)的研究揭示花岗岩主要有3种来源：一是t(下标 2DM)介于1.2~1.6 Ga的中元古代地壳源区；二是t(下标 2DM)介于0.7~1.6 Ga的中新元古代地壳混合源区；三是t(下标 2DM)与岩石形成年龄接近或略大的以亏损地幔新生地壳为主的地壳源区。后者变形很弱，LA-ICPMS锆石U-Pb定年揭示其形成时代为320~352 Ma。晚泥盆世－石炭纪花岗岩主要源区亦有3种：一是t(下标 2DM)介于1.0~1.7 Ga的中新元古代混合源区；二是t(下标 2DM)约为0.4 Ga左右的早古生代新生地壳源区；三是早石炭世早期与岩浆形成年龄一致的亏损地幔和古老地壳混合源区。那拉提早石炭世花岗闪长岩当εHf (t)为最高的正值时(14.21)，其t(下标 2DM)与岩石形成年龄一致，表明在349 Ma时存在一次地幔物质的加入。综合西天山地区区域地质演化和花岗岩的野外地质特征、时代格架、锆石Hf同位素揭示的源区特征，认为那拉提地区现划分的古元古代地层中存在0.7~0.8 Ga、1.6~1.8 Ga中新元古代增生地壳。石炭纪花岗岩为碰撞后大陆伸展作用的产物，早石炭世大规模地幔物质的加入代表了大陆伸展作用的强烈发育期。
Paleozoic granitic magmatism is well developed in Nalati area of West Tianshan Mountains. Based on field work, petrographic studies and LA-ICPMS zircon dating, the authors divided granitic magmatism in Nalati area into two stages. The first stage is from Late-Ordovician to Late-Devonian, and the granitoids formed in this stage have experienced a strong deformation and developed gneissic schistosity and large exposed areas of diorite, quartz diorite, granodiorite and monzogranite intrusions. From west to east, they include Senmutasi intrusion, Haerwenke intrusion, Kekesuhe intrusion and Nalati intrusion. Granitoids of this kind in the study area were mainly formed between 485 Ma and 366 Ma and constitute the main body of the granitoids in Nalati. The second stage is Carboniferous and the granitoids formed in this stage are located in the north of the first stage granitoids or intruded into the first stage granitoids, and their exposed area is usually small. They have only undergone a weak or no deformation. Along the NEE-trending Nalati tectonic belt, there exists the feature that the formation ages of the granitoids gradually become younger from west to east, and become younger from south to north across the Nalati tectonic belt. Zircon Hf isotope studies reveal that the granitoids formed between Late-Ordovician and Late-Devonian in this area have three kinds of sources. The first (t(subscript 2DM)=1200~1600 Ma) is Mesopro-terozoic crust, the second (t(subscript 2DM)=700~1600 Ma) is Meso-and Neo-Proterozoic crust, and the third (the values of t(subscript 2DM) are similar to or a little older than the formation ages of the granitoids) is a juvenile crust. Carboniferous granitoids also have three kinds of sources. The first (t(subscript 2DM)=1000~1700 Ma) is a Meso and Neo-Proterozoic mixed source, the second (t(subscript 2DM)≈400 Ma) is an Early Paleozoic juvenile crust, and the third is a mixed region of Early-Carboniferous depleted mantle and ancient crusr. Zircon Hf isotope researches on granitoids suggest that there exits a Meso- and Neo-Proterozoic crustal growth event in Nalati area. The Paleoproterozoic basement considered by present researchers may include Neoproterozoic, Mesoproterozoic, Paleoproterozoic and Archean materials, and its internal structure is so complex that we should do much future work on it. Chronological framework and deformation characteristics of granitoids suggest that the Late Devonian period may represent the end of the Paleozoic ocean-continent transition in the Tianshan Mountains. Zircon Hf isotope studies suggest that there were two events of mantle material addition that took place in Early Silurian and Early Carboniferous respectively. The former was a period of the Tianshan Paleozoic ocean subduction, and during this time the granitoids were formed by partial melting of a juvenile crust with the addition of some mantle materials. The latter represented a crustal growth event in Early Carboniferous, which suggests an influx of some mantle materials in a strong continental extension stage after a collisional orogeny.