Petrochemical constraints for dual origin of garnet peridotites from the Dabie‐Sulu UHP terrane, eastern‐central China
Petrochemical constraints for dual origin of garnet peridotites from the Dabie‐Sulu UHP terrane, eastern‐central China
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DOI：
10.1046/j.1525-1314.2000.00248.x
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发表时间：
2000-03
影响因子：
3.4
通讯作者：
Zhang;Liou;Yang;Yui
中科院分区：
文献类型：
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作者：
Zhang;Liou;Yang;Yui
石榴石橄榄岩以透镜体、块状或层状形式出现在大别-苏鲁超高压（UHP）岩中的麻粒-角闪岩相片麻岩中，并含有含柯石英的榴辉岩。根据产状和岩石化学特征，识别出两种不同类型的石榴橄榄岩。A型幔源橄榄岩起源于：（1）俯冲带上方的地幔楔，（2）俯冲板片的下盘地幔，或（3）在超高压变质作用之前侵位在地壳深处的古老地幔碎片，而B型地壳橄榄岩和辉石岩是俯冲之前作为岩浆侵入大陆地壳的镁铁质-超镁铁质杂岩的一部分。大部分A型橄榄岩来自亏损地幔，具有地幔岩石的岩石化学特征;然而，部分橄榄岩的Sr和Nd同位素组成在俯冲和/或折返过程中受到地壳混染的影响。B型橄榄岩和辉石岩为堆晶构造，部分经历了地壳交代和混染作用，表现为高87 Sr/86 Sr比值（0.707-0.708）、低εNd（t）值（−6 ~ −9）和低δ 18 O值（+2.92 ~+4.52）。这两种类型的石榴橄榄岩经历了多阶段的重结晶;其中一些记录了重结晶历史。高峰变质作用的高P-T估计值（760-970 °C和4.0-6.5±0.2 GPa）表明，幔源和壳源超镁铁岩都俯冲到了>100 km的深度（最深可能≥180-200 km），并在地热梯度极低（<5 °C km−1）的俯冲带中经历了超高压变质作用。
Garnet peridotites occur as lenses, blocks or layers within granulite–amphibolite facies gneiss in the Dabie‐Sulu ultra‐high‐pressure (UHP) terrane and contain coesite‐bearing eclogite. Two distinct types of garnet peridotite were identified based on mode of occurrence and petrochemical characteristics. Type A mantle‐derived peridotites originated from either: (1) the mantle wedge above a subduction zone, (2) the footwall mantle of the subducted slab, or (3) were ancient mantle fragments emplaced at crustal depths prior to UHP metamorphism, whereas type B crustal peridotite and pyroxenite are a portion of mafic–ultramafic complexes that were intruded into the continental crust as magmas prior to subduction. Most type A peridotites were derived from a depleted mantle and exhibit petrochemical characteristics of mantle rocks; however, Sr and Nd isotope compositions of some peridotites have been modified by crustal contamination during subduction and/or exhumation. Type B peridotite and pyroxenite show cumulate structure, and some have experienced crustal metasomatism and contamination documented by high 87Sr/86Sr ratios (0.707–0.708), low εNd(t) values (−6 to −9) and low δ18O values of minerals (+2.92 to +4.52). Garnet peridotites of both types experienced multi‐stage recrystallization; some of them record prograde histories. High‐P–T estimates (760–970 °C and 4.0–6.5±0.2 GPa) of peak metamorphism indicate that both mantle‐derived and crustal ultramafic rocks were subducted to profound depths >100 km (the deepest may be ≥180–200 km) and experienced UHP metamorphism in a subduction zone with an extremely low geothermal gradient of <5 °C km−1.