A post-collisional magmatic plumbing system: Mesozoic granitoid plutons from the Dabieshan high-pressure and ultrahigh-pressure metamorphic zone, east-central China
A post-collisional magmatic plumbing system: Mesozoic granitoid plutons from the Dabieshan high-pressure and ultrahigh-pressure metamorphic zone, east-central China
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DOI：
10.1016/s0024-4937(98)00043-7
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发表时间：
1998-12
期刊：
影响因子：
3.5
通讯作者：
Changqian Ma;Zhichang Li;C. Ehlers;Kunguang Yang;Renjing Wang
中科院分区：
文献类型：
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作者：
Changqian Ma;Zhichang Li;C. Ehlers;Kunguang Yang;Renjing Wang
大别山高压-超高压变质带中发现了三组中生代钾玄质或高钾钙碱性侵入岩，它们与板片滑脱、岩浆底侵和穹隆作用有关。第Ⅰ组为板裂型，由晚三叠世（约210 Ma）的镁铁质二长闪长岩组成。它具有中到高Sr，低Rb和Ba丰度，中等轻稀土元素（LREE）/重稀土元素（HREE）和K/Rb比值。Ⅱ组为底侵型，主要由中侏罗世-早白垩世（160-120 Ma）的角闪石石英二长岩、黑云二长花岗岩和正长花岗岩组成，具有较高的LREE/HREE和K/Rb比值，Sr和Ba的富集范围大，Rb的富集范围小。第Ⅲ组为穹隆型，以白垩纪（125-95 Ma）花岗质岩株和花岗斑岩为代表。与第二组相比，其Rb、Y、HREE丰度高，Sr、Ba丰度低，LREE/HREE和K/Rb比值低。所有群具有相似的Nd和Sr同位素组成。其中，大别山地区最丰富的是碰撞后花岗岩类岩浆活动（Ⅱ组），其年龄为160 ~ 120 Ma，晚于高压和超高压变质作用的年龄为245 ~ 220 Ma。碰撞后的花岗岩类岩体最初侵位在不同的水平，从中地壳到近地表。研究表明，花岗岩类的全岩化学随结晶压力而系统地变化。如K2 O、标准Or、Rb和Zr随压力下降而增加，而Ba、Nb、Nd、Yb、MnO和标准An则随压力上升而减少。有人建议，上升分化，浮力液体，结合分馏与同化（AFC）配对，是负责的垂直变化。地质、地球化学和岩石学资料表明，I组可能是由于板片断裂导致富集的陆下岩石圈地幔部分熔融而形成的。第II类岩石可能主要来自地壳同化/熔融和幔源岩浆的分离结晶，而第III类岩浆可能来自大别杂岩的深熔作用，并在热穹隆环境中高度演化。晚三叠世-早侏罗世的板片断裂可能是超高压岩石通过地幔折返的原因。大量花岗质侵位和一段快速剥蚀表明，岩浆底侵和膨胀可能在中侏罗世-早白垩世大别山快速折返中发挥了作用。
Three groups of Mesozoic shoshonitic or high-K calc-alkaline intrusive rocks are identified in Dabieshan high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic zone, east-central China and they are related to: (I) slab breakoff; (II) magmatic underplating; and (III) doming. Group-I, the slab breakoff-type, consists of late Triassic (∼210 Ma) mafic monzodiorites. It has moderate to high Sr, and low Rb and Ba abundances, and moderate light rare earth element (LREE)/heavy rare earth element (HREE) and K/Rb ratios. Group-II, the underplating-type, consists mainly of middle Jurassic–early Cretaceous (160–120 Ma) hornblende quartz monzonitic, biotite monzogranitic, and syenogranitic plutons, characterized by relatively high LREE/HREE and K/Rb ratios, and by a large range in concentration of Sr and Ba, coupled with much smaller range in Rb. Group-III, the doming-type, is represented by Cretaceous (125–95 Ma) granitic stocks and granitic porphyry. Compared with group-II, it has high Rb, Y and HREE abundances, low Sr and Ba abundances and low LREE/HREE and K/Rb ratios. All groups have similar Nd and Sr isotopic compositions. Among the three groups, post-collisional granitoid magmatism (group-II) with ages of 160 to 120 Ma, post-dating the HP and UHP metamorphism at 245 to 220 Ma, is the most abundant in the Dabieshan area. The post-collisional granitoid plutons were initially emplaced at different levels ranging from mid-crust to near-surface. This study shows that the whole-rock chemistry of the granitoids vary systematically with crystallization pressures. For example, K2O, normative Or, Rb and Zr show the strongest increase with decreasing pressure, whereas Ba, Nb, Nd, Yb, MnO, and normative An decrease upward in the Dabie Block. It is suggested that ascent of differentiated, buoyant liquids, combined with fractionation paired with assimilation (AFC), is responsible for the vertical variation. Geological, geochemical and petrological data indicate that group-I could have been generated by partial melting of enriched subcontinental lithosphere mantle due to slab breakoff. Group-II rocks could have been produced mainly from crustal assimilation/melting and fractional crystallization of mantle-derived magmas, whereas group-III magma could have derived from anatexis of the Dabie complex and was highly evolved in a hot doming setting. The late Triassic-early Jurassic slab breakoff may be responsible for the exhumation of UHP rocks through the mantle. The voluminous granitic emplacement together with an episode of rapid denudation suggests that magmatic underplating and inflation could have played a role in the Middle Jurassic–Early Cretaceous rapid exhumation of Dabieshan.