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首页> 外文期刊>Journal of Asian earth sciences >Constraints from zircon Hf-O-Li isotopic compositions on the genesis of slightly low-δ~(18)O alkaline granites in the Taohuadao area, Zhejiang Province, SE China
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Constraints from zircon Hf-O-Li isotopic compositions on the genesis of slightly low-δ~(18)O alkaline granites in the Taohuadao area, Zhejiang Province, SE China

机译：中国浙江省桃花岛地区锆石Hf-O-Li同位素组成对δ-（18）O稍低的碱性花岗岩成因的约束

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摘要

The origins of the Mesozoic low-delta O-18 A-type granites in eastern China are quite controversial. A new "low-delta O-18" granite, the Taohuadao alkaline (A(1)-type) granite from the Zhoushan archipelago, offshore from Zhejiang, is reported in this study. Integrated in situ analyses of Hf-O-Li isotopes in zircon are used to provide insights on the petrogenesis and evolution of this granite, and low-delta O-18 granites in general. Three types of zircon have been recognized in Taohuadao samples: zircon with sector/oscillatary zoning (Type 1), disturbed zoning (Type 2) and extremely high-U (Type 3) grains. Data are carefully screened for radiation damage and post-magmatic alteration and only Type 1 zircon represents a magmatic origin. In Type 1 zircons, Type la with normal mantle delta O-18(zrn) value of 5.23 parts per thousand has a positive delta Li-7 value (2.0 +/- 3.3 parts per thousand), while Type 1b has a delta O-18 value of similar to 4.81 parts per thousand and highly variable negative delta Li-7 values (-31.9 parts per thousand to -0.3 parts per thousand). In contrast to the O-Li isotopes, zircon is quite homogeneous in terms of Hf isotopes (epsilon Hf-(t) = - 3.5 +/- 1.7), suggesting that the Taohuadao A(1)-type granites were derived from a simple source. Our results show that the magmatic zircon delta O-18(zrn) values are negatively correlated with Hf content but positively correlated with Zr/Hf ratio in zircon, which indicates that fractional crystallization plays an important role in producing the slightly low delta O-18 signature. The anhydrous nature of the As-type granites suppressed the crystallization of biotite and hornblende, while low oxygen fugacity is responsible for insufficient precipitation of magnetite, both of which drive the later differentiates towards slight depletion in O-18. Therefore, we suggest a ferrogabbro-type fractional crystallization may have played a major role in producing the chemical and oxygen isotopic variations of the Taohuadao A(1)-type granite and accounts for similar to 0.50 parts per thousand O-18-depletion. The extremely depleted delta Li-7 in Type 1b zircons is most likely a result of diffusion-driven fractionation. Mantle-like O-Li isotopes and very low Li abundance in zircon suggest a predominantly mantle origin with little, if any, involvement of continental crustal materials in the granite. Thus the Taohuadao A(1)-type granite was most likely produced by intensive fractionation of reduced, anhydrous basaltic melts, rather than by mixing of mantle- and crustally-derived melts as previously thought.
机译：中国东部中生代低三角洲O-18 A型花岗岩的成因颇有争议。这项研究报道了一种新的“低δO-18”花岗岩，即浙江离岸舟山群岛的桃花岛碱性（A（1）型）花岗岩。锆石中Hf-O-Li同位素的综合原位分析可用于深入了解该花岗岩和一般低δO-18花岗岩的岩石成因和演化。桃花岛样品中已识别出三种类型的锆石：具有扇形/振荡带状（1型），干扰带状（2型）和极高U（3型）晶粒的锆石。对数据进行了仔细的筛选，以检查是否存在辐射损害和后岩浆蚀变，只有1型锆石代表岩浆成因。在类型1的锆石中，正常披风delta O-18（zrn）值为5.23千分之一的类型1a具有正的Li-7值（2.0 +/- 3.3千分之一），而类型1b的delta O- 18个值类似于4.81千分之一和高度可变的负增量Li-7值（-31.9千分到-0.3千分）。与O-Li同位素相反，锆石在Hf同位素方面非常均匀（εHf-（t）=-3.5 +/- 1.7），这表明桃花岛A（1）型花岗岩源自简单的资源。我们的研究结果表明，岩浆锆石的δO-18（zrn）值与Hf含量呈负相关，但与锆石中的Zr / Hf比值呈正相关，这表明分步结晶在产生稍低的δO-18中起着重要作用。签名。 As型花岗岩的无水性质抑制了黑云母和角闪石的结晶，而低氧逸度则导致磁铁矿沉淀不足，这两者都促使后者在O-18中向轻度贫乏分化。因此，我们建议铁硼铁型分级结晶可能在桃花岛A（1）型花岗岩的化学和氧同位素变化产生中起主要作用，占O-18损耗的千分之0.50相似。 1b型锆石中δ7极耗尽的极有可能是扩散驱动分级分离的结果。锆石中的类似地幔的O-Li同位素和极低的Li含量表明，地幔的起源主要是花岗岩中几乎没有大陆地壳物质参与（如果有的话）。因此，桃花岛A（1）型花岗岩最有可能是通过对还原的无水玄武岩熔体进行密集分馏而产生的，而不是像以前认为的那样通过混合地幔和地壳衍生的熔体来生产的。

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