华北地区晚中生代镁铁质岩浆作用及其地球动力学背景

华北陆块及周缘地区晚中生代镁铁质岩浆岩的元素-同位素地球化学特征显示岩石圈地幔的区域不均一性。华北内部为古老而富集的EM1型岩石圈地幔,主要岩性为弥散状金云母相橄榄岩;华北北缘的岩石圈地幔相对华北内部在化学成分上饱满,在微量元素特征上高度富集LILE、LREE和亏损Nb-Ta和U-Th,在同位素组成上相对高87Sr/86Sr(i)和εNd(t),为受到再循环古老陆壳组分改造的富集型地幔;华北陆块南缘的岩石圈地幔以高87Sr/86Sr(i)和低εNd(t)为特征,与深俯冲大陆地壳改造作用 (残留陆壳板片和熔体 -地幔反应等多种形式 )密切相关。发育在华北陆块及周缘地区的晚中生代镁铁质岩浆作用形成于岩石圈伸展 -减薄的统一动力学背景。考虑到晚中生代华北陆块受到了来自周缘陆块相互作用,如西南特提斯域构造演化、印支陆块和西伯利亚板块的侧向挤压作用和古太平洋板块迅速向北运动引起的走滑拉分作用的共同影响, 我们倾向认为这些板块边界作用引起的板内效应可能是导致华北岩石圈地幔晚中生代广泛熔融和岩石圈减薄的重要动力来源.

LATE MESOZOIC MAFIC MAGMATISM FROM THE EASTERN PART OF NORTH CHINA BLOCK AND ITS ADJACENT REGIONS: MAGMA GENESIS AND GEODYNAMIC IMPLICATIONS FOR LITHOSPHERIC THINNING

A comprehensive study on geochemical comparison between the different units in the eastern part of North China Block (NCB) is made for understanding the origin of the intensive mafic magmatism in late Mesozoic. Major elements, trace elements and Sr-Nd isotope data of these igneous rocks suggest chemical and isotopic heterogeneities of the subcontinental lithospheric mantle (SCLM) beneath different tectonic units. The SCLM of the NCB interior is characterized by long-term enrichment in LILE and LREE but depletion in HFSE with EM1-like Sr-Nd isotopic signatures. Such a kind of lithospheric mantle is chemically refractory and is mainly composed of disseminated phlogophite-bearing harzburgites that are common in on-cratonic regions in the globe. In contrast with that in the NCB interior, beneath the northern NCB existed a chemically fertile mantle of LILE- and LREE-enrichment and Nb-Ta and Th-U depletion. This kind of SCLM has higher ()~(87)Sr/~(86)Sr(i) and ε_(Nd)(t) than that of the NCB interior, and is interpreted to have been modified by recycled lower continental crustal materials related to the closure of the Paleo-Asian Ocean during late Paleozoic to early Mesozoic. The SCLM of the southern NCB has the highest ()~(87)Sr/~(86)Sr(i) and the lowest ε_(Nd)(t) value, and chemically transitional between the NCB interior and northern NCB. Formation of the lithospheric mantle is closely associated with the modification (e.g., residual crustal relicts, source mixing, melt metasomatism and melt-mantle interaction) from the lower-middle crust of the subducted Yangtze Block during the Triassic collision between the NCB and Yangtze Block, despite the discrepancy in the detailed enrichment processes. Based on the temporary consistency and spatial distribution, a lithospheric extension-thinning model is proposed to explain the petrogenesis of all late Mesozoic mafic rocks in the eastern NCB. This tectonic model plays emphasis on the resultant effect from the surrounding plate interaction, including the contemporaneous rapid northward movement of the paleo-Pacific Plate, compressional forces from the Siberian Plate and the Tethyan tectonic domain and possibly the Indo-China block. The resultant force triggered lithospheric extension and asthenospheric upwelling and thus decompressional melting of the enriched mantle sources, and the ultimate extensive emplacement of late Mesozoic mafic magmas.