鲁西中生代高Mg闪长岩的成因: 年代学与岩石地球化学证据

鲁西高Mg闪长岩分布较为广泛, 莱芜铁铜沟岩体和淄博金岭岩体是其典型代表.铁铜沟岩体早期苏长辉长岩和晚期辉石闪长岩、金岭黑云母闪长岩的LA-ICPMS U-Pb谐和加权平均年龄分别为(131.4±4.9)Ma(n=15)、(134.5±2.3) Ma(n=13)和(132.8±4.2)Ma(n=12), 它们代表了岩体的侵位结晶年龄——早白垩世.铁铜沟岩体晚期辉石闪长岩中浑圆状锆石的LA-ICPMS U-Pb谐和年龄为(2 513±54)Ma(n=8), 代表了华北克拉通基底的存在.该类岩石以高Mg# (> 60)、富钠、富含轻稀土元素、贫重稀土元素以及高场强元素(Nb、Ta、Zr、Hf)的明显亏损为特征, 显示埃达克岩(adakite) 的特征.该类岩石中地幔橄榄岩包体的广泛存在以及高Mg的特征表明原始岩浆的地幔成因, 而Sr-Nd同位素组成(I_sr为0.704 75-0.707 72和ε_Nd(t)值为-3.95--13.30)和高场强元素(Nb、Ta、Zr、Hf)的亏损以及太古代继承锆石的存在均暗示岩浆源区应有陆壳物质的涉入.铁铜沟岩体和金岭岩体闪长岩成分的差异应归因于岩浆源区性质的差异和部分熔融程度的不同.基于该类岩石的地球化学和所含包体, 同时结合华北克拉通东部中生代早期岩石圈演化历史, 可以认为鲁西中生代早白垩世高Mg闪长岩的形成应为拆沉的岩石圈(地幔+下地壳)与软流圈混熔的产物. 

Petrogenesis of the Mesozoic High-Mg Diorites in West Shandong: Evidence from Chronology and Petro-geochemistry

High-Mg diorites occur widely in western Shandong: the Tietonggou and Jinling intrusions are typically representative. The weighted mean 206pb/238U ages from LA-ICPMS zircon U-Pb dating results for early norite-diorite, late pyroxene-diorite from the Tietonggou intrusion and biotite-diorite from the Jinling intrusion are (131. 4±4. 9)Ma(n = 15), (134. 5±2. 3)Ma(n=13), (132. 8±4. 2)Ma(n=12), respectively, implying that they were formed in the Early Cretaceous. The weighted mean 207Pb/206Pb age for round zircons from late pyroxene-diorite from the Tietonggou intrusion is (2 513±54)Ma(n=8), suggesting that the basement of the North China craton should exist in the research area. The high-Mg diorites are characterized by enrichment in Mg#, Na, and light rare earth elements (LREE). They are poor in heavy rare earth elements (HREE) and high field strength elements (HFSE), being similar to adakite. The occurrence of the mantle peridotite xenoliths and the high-Mg feature for these intrusive rocks imply that their primary magma should be derived from the upper mantle. However, Sr-Nd isolopic compositions (I_sr: 0.704 75 - 0. 707 72; ε_Nd(t) values: -3. 95 to - 13. 30), depletion in HFSE, and the occurrence of the Archean inherited zircons suggest that crustal materials be involved in the derivation of primary magma The compositional difference between the diorites from the Tietonggou and Jinling intrusions could be attributed to magma sources and partial melting degrees. It is considered that the Early Cretaceous high-Mg diorites were formed by the mixed melting of the delaminated lithosphere (lithospheric mande+crust) and asthenosphere, based on their geochemistry, the mantle-derived xenoliths, and the Early Mesozoic lithospheric evolutionary history of the eastern North China craton.