南海西部晚中生代岩浆弧:KP1-1-1井碎屑锆石约束

晚中生代是古太平洋板块俯冲和南海北部岩浆弧发育时期，开展珠江口盆地KP1-1-1井钻遇的浅变质砂岩的近源沉积研究有助于揭示岩浆弧源区的构造特点.根据LA-ICPMS碎屑锆石分析，KP1-1-1浅变质砂岩中存在129~155 Ma、155~172 Ma和172~196 Ma三个年龄组分，这一特点与区域上构造岩浆事件一致.碎屑岩浆锆石流体活动性元素富集（如U和Th），高场强元素亏损（如Nb、Hf和Ti）；元素组成U/Yb（0.34~3.92）、Sc/Yb（0.48~2.28）、Hf/Th（14.4~186.6）和Th/Nb（24.3~462.7）具有大陆岩浆弧特点；计算的锆石Ti温度为551~786℃，表明属于低温弧岩浆作用.碎屑岩中172~196 Ma锆石组分记录了东沙-大仑-雁荡岩浆弧向西南的进一步延伸，与侏罗纪早期古太平洋斜向俯冲到华南陆块之下有关.155~172 Ma岩浆锆石与古板块强烈俯冲有关，对应于华南165~150 Ma大规模花岗质岩浆活动.129~155 Ma时期板块俯冲减弱或俯冲带后撤，可与浙闽同时期板内强烈火山活动对比.碎屑岩中测得最年轻年龄为128.8 Ma，表明KP1-1-1钻遇浅变质砂岩形成时代晚于128 Ma，应属于白垩系，不是传统上认为的下古生界岩系. 

Late Mesozoic Magmatic Arc: Constraints from Well KP1-1-1 Detrital Zircon U-Pb Data,South China Sea

The Late Mesozoic is an important tectonic episode for Paleo-Pacific slab subduction and magmatic arc evolution in the South China Sea. The low-metamorphosed sandstone drilled by KP1-1-1 belongs to a proximal deposition which helps to reveal tectonic characteristics of the magmatic arc provenance. Based on LA-ICPMS detrital zircon analysis, age components of 129-155 Ma, 155-172 Ma, and 172-196 Ma are identified from KP1-1-1 sandstone in the western Pearl River Mouth Basin, in accordance with regional tectono-magmatic events. These magmatic zircons which derive from arc-related magmatic source are characterized by low-temperature (551-786℃), enrichment of fluid-mobile elements (U, Th) and depletion of high field-strength elements (Nb, Hf and Ti). They are characterized by high element ratios of U/Yb (0.34-3.92), Sc/Yb (0.48-2.28), Hf/Th (14.4-186.6), and Th/Nb (24.3-462.7), indicating the related magmatism in a continental arc environment. The age component of 172-196 Ma from the sandstone registers the Early Jurassic Dongsha-Talun-Yandang magmatic arc further stretching southwest in East to South China Seas, as a result of oblique initial subduction of the Paleo-Pacific slab. The major component of 155-172 Ma reveals an intensified slab subduction and arc-related magmatism in South China Sea, comparable to voluminous granitic magmatism (165-150 Ma) in South China. The minor component of 129-155 Ma found, however, implies a reduced arc-related magmatism due to slow subduction or slab rollback, sharply in contrast to intensified rhyolite volcanism (120-160 Ma) developed in Zhejiang to Fujian coastal areas. The minimum U-Pb age 128.8 Ma of detrital zircon defines the stratigraphic age of KP1-1-1 sandstone as Cretaceous, not the latest Late Proterozoic to Early Paleozoic as traditionally treated.