Origin of the Low δ18O Signals in Zircons from the Early Cretaceous A-Type Granites in Eastern China: Evidence from the Kulongshan Pluton
Origin of the Low δ18O Signals in Zircons from the Early Cretaceous A-Type Granites in Eastern China: Evidence from the Kulongshan Pluton
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DOI：
10.1007/s12583-021-1515-y
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发表时间：
2021-09
影响因子：
3.3
通讯作者：
Wenbo Fan;Neng Jiang;M. Zhai;Jun-Cheng Hu
中科院分区：
文献类型：
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作者：
Wenbo Fan;Neng Jiang;M. Zhai;Jun-Cheng Hu
中国东部早白垩世A型花岗岩中锆石低δ 18 O信号的成因一直存在争议。18 O亏损的特征是继承于高温热液蚀变源岩还是侵位后水-岩相互作用的结果尚不确定。本文对华北北方库龙山130 Ma A型花岗岩的锆石氧同位素进行了研究。根据CL图像中锆石的发光强度和内部结构，可将其分为5种类型。δ 18 O值也有不同类型的变化，且与U、Th含量及伴随的累积α衰变剂量呈负相关，表明其δ 18 O值可能在辐射损伤累积后受到大气降水-岩石相互作用的不同程度的改变。共存矿物对之间的氧同位素平衡计算进一步证实了这一观点。推测只有那些δ 18 O值比正常地幔稍高的受影响最小的锆石才保存了岩浆氧同位素。结合其他证据，提出A型花岗岩为下地壳成因，不应援引高温热液蚀变源区。这一观点也适用于其它许多具有类似锆石行为的白垩纪A型花岗岩。
The origin of low δ18O signals in zircons from the Early Cretaceous A-type granites in eastern China has long been disputed. It is uncertain whether the18O-depleted features were inherited from high-temperature hydrothermal altered source rock or resulted from water-rock interaction after emplacement. In this paper, zircon oxygen isotopes in the ∼130 Ma Kulongshan A-type granites in the northern North China Craton are analyzed. The zircons could be subdivided into 5 types based on their luminescent intensity and internal structures in CL images. Their δ18O values also vary in different types and show negative correlation with U and Th contents and accompanying cumulative α-decay doses, implying that their δ18O values may have been modified to various degrees by meteoric water-rock interaction after the accumulation of radiation damage. The idea is further confirmed by oxygen isotopic equilibrium calculation between co-existing mineral pairs. It is inferred that only the least-influenced zircons, with slightly elevated δ18O values than normal mantle, have preserved the magmatic oxygen isotopes. In combination with other evidences, it is proposed that the A-type granites are lower-crustal-derived, unnecessarily invoking a high-temperature hydrothermal altered source. The proposition is applicable to many other Cretaceous A-type granites that have similar zircon behaviors.