摘要
对西藏西部日土县城以南一拉梅拉山口一带的花岗岩体开展了详细的岩相学、岩石地球化学和锆石u—Pb年代学及Hf同位素研究。所有样品铝饱和指数A／CNK集中在O．76～1．0之间，为准铝质类型。CIPW标准矿物组合为Q＋Or＋Ab＋An4-Di（或c）＋Hy。在稀土元素配分图中呈现出右倾缓倾斜型的特征，轻稀土元素富集并出现较强的分馏作用，重稀土元素无分馏-轻微分馏。8Eu在0．56一O．99范围之间，属于铕亏损型。大离子亲石元素出现分化，富集Rb、Pb、Th而亏损K、Ba，高场强元素Nb、Ta、Ti等明显亏损。获得钾长花岗岩、二长花岗岩及花岗闪长岩中岩浆结晶锆石的LA．ICP．MSU-Pb年龄分别为：79．4±0．4Ma、81．0±0．5Ma和81．3±0．5Ma，结合锆石稀土元素和岩浆振荡环带特征及Th／u比值，上述年龄结果可代表岩石的结晶年龄，表明该套岩体为晚白垩世侵位的大型岩基。两件样品的锆石均具有正的Hf同位素初始比值6Hf（t），两阶段Hf模式年龄（tD。）分别介于547．5—658．0Ma、523．4—710．2Ma之间。分析认为该套岩体的物质来源应该为富角闪石的下地壳，可能为幔源岩浆首先侵入到地壳基底岩石中形成新生地壳，然后在温度约为700～800~C之间、压力〈8kbar且富合流体的影响下，这种既有新生地壳又有古老基底地壳构成的混合地壳发生部分熔融而形成。这一结论与野外宏观露头上岩体中大关俊雷等：北冈底斯带日土县．拉梅拉山口花岗岩体的岩石地球化学特征、锆石u—Pb测年及Hf同位素组成量发育暗色微粒包体等直接岩石学证据相佐证。结合区域构造演化及岩体所处的大地构造位置，该套花岗岩体应该形成于洋壳闭合时的碰撞造山过程，其形成与侵位与北侧班公湖一怒江结合带的构造演化有成因上的联系，是班公湖一怒江特提斯洋向南的俯冲碰撞的产物。
Granite samples from Ritu County-Lameila pass area of western Tibet were selected for a detailed geochronological and geochemical analysis to study its petrogenesis. All samples were metaluminous （0. 76 〈 A/CNK 〈 1.0）, and had Q ＋ Or ＋ Ab ＋ An ＋ Di （or C） ＋ Hy in CIPW composition. They also showed in right steeply-deviation from LREE to HREE with obviously negative Eu anomaly （SEu =0. 56 ~0. 99）, enriched in Rb, Pb, Th and depleted in K, Ba, especially HFSE （Nb, Ta, Ti）. LA-ICP-MS U-Pb zircon ages for moyite, monzogranite and granodiorite are 79.4 ± 0. 4Ma, 81.0 ±0. 5Ma and 81.3 ± 0. 5Ma respectively, indicating that they were all formed in Late Cretaceous. Zircon Hf isotopic compositions of two samples showed positive Hf isotopic initial ratio and had 547.5 ~ 658.0Ma, 523.4 ~ 710. 2Ma of two stage model ages respectively. Petrochemistry indicated that primitive material of the granite came from hornblende-enriched lower crust, which were formed by old crust and neo-crust mixing and partial melting in the condition of 700 ~ 800℃, 〈 8kbar and fluid-enriched. Such conclusion agreed with the petrology evidence that mafic microgranular enclaves （MME） were found in its outcroup. The granite were formed by granitic magmatisms during the oceanic crust orogenic subducting, as the magmatic response to the Bangong-Nujiang Tethys evolution.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2014年第6期1666-1684,共19页
Acta Petrologica Sinica
基金
国家自然科学基金项目(41273047)
中国地调局青藏专项(1212011086068
1212011121256)联合资助
