Geochronological, whole-rock elemental and Sr-Nd isotopic and apatite geochemical studies of Early Yanshanian (Jurassic) intrusives from Nanling Mountains area, South China and their tectonic implications

• Main Authors: Pei-Shan Hsieh, 謝佩珊
• Other Authors: Cheng-Hong Chen
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/29509494171735610159

博士 === 國立臺灣大學 === 地質科學研究所 === 96 === The widespread Mesozoic intrusives in South China were emplaced in three main periods: Triassic (Indosinian), Jurassic (Early Yanshanian) and Cretaceous (Late Yanshanian). Lithologically, granites predominate over basic and intermediate rocks, and volumetrically, Early Yanshanian granitoids are the most widely exposed in the Cathaysia Block (approximately 50% of the total surface area of Mesozoic granitoids). In this study, zircon U-Pb ages, whole-rock and apatite geochemical and Sr-Nd isotopic analyses were carried out to provide better constraints on the temporal-spatial distribution and the petrogenetic model for Early Yanshanian intrusives. Furthermore, the shape for Early Yanshanian granitoids in Nanling Mountains was analyzed to understand the relationship between granitic bodies and regional strike-slip faults and the Mesozoic tectonic evolution of the South China. The Early Yanshanian intrusives were basically formed in three stages: 190-175 Ma (early), 165-150 Ma (main), and 145-135 Ma (late). They consist dominantly of high-K calc-alkaline granitic batholiths that generally show variable concentrations of LILE, significant depletion in Nb, Sr, Ti, Zr and P in spidergram and crustal-derived Sr-Nd isotope compositions (ISr=0.712~0.717; eNd(T)=-8.8~-11.6). Those formed as small plutons are medium-K calc-alkaline gabbros and shoshonitic syenites which have insignificant depletion in Nb and Ta and more depleted Sr-Nd isotope compositions (ISr=0.704~0.708; eNd(T)=+3.0~-2.6) to granitoids. Apatite geochemistry has been suggested as a sensitive indicator of the crystallization environment and the distribution of trace elements in apatite can, therefore, be used to distinguish S-type and I-type granites. Such an application to the Triassic Darongshan and Cretaceous Fuzhou-Zhangzhou granites in S China is generally valid. However, it is less applicable to the intermediary type Jurassic Nanling Mountains granites. REE distribution patterns of apatite strongly depend on the host rock ASI value and Nd isotope composition, and the majority of NLM apatites can define a kind of pattern that is distinguishable from Darongshan and Fuzhou-Zhangzhou apatites, or Lachlan Folded Belt S- and I-type apatites. Furthermore, Sr and Nd isotope evidence indicates that two possible crustal sources for Nanling Mountains granitic magmas can be represented predominantly by the pre-Mesozoic (mainly Caledonian) and subordinately by the pre-Yanshanian (mainly Indosinian) granitic rocks. Intrusives of the early stage (A-type granites and gabbro) are less and concentrated in the southern Jiangxi and Shi-Hang zone. Intrusives (mainly granitoids) of the main stage are widespread in the Nanling Mountains region and are the essence of the Early Yanshanian magmatism. The intensive main stage magmatism formed within a short time span suggests that the SE China was largely under an extensional environment. Intrusives of the late stage are scarce, indicating that the Late Yanshanian magmatism was waned. Presence of syenites, mafic dikes and even bimodal volcanics is a sign of mantle-derived melts injected into the localized extensional areas. It is worth to note that some specialized areas, like southern Jiangxi, central Guangdong and Shi-Hang zone, are characterized by record of repeated mantle-derived magmatism which was generated under extensional environment. Therefore, in situ crustal melting is the most plausible model to account for the genesis of Jurassic Nanling Mountains granites. The existence of sporadic alkali rock, gabbro and mafic dike indicates local mantle input at places where crustal extension prevailed. Extensional or rifting tectonism causing uprising of mantle-derived magmas to underplate beneath the crust can provide the heat source for remelting of overlying crust materials. It is obvious that the distribution of intrusives in the Nanling Mountains granites were closely related to regionally deep, concealed faultings that underwent high lateral displacements. The high displacement velocity is suggested to be the main factor to affect intrusions with normal asymmetry and same trending with the local principal stretching direction. The Indosinian peraluminous granites in Cathyasia Block are thought to be the Triassic syn-orogenic products. It was followed by a magmatically inactive perioid for ~15 Myr before onset of the early-stage Early Yanshanian event, probably reflecting the change of tectonic setting from compressional to extensional. Consequently, the pre-existing NE-trending sinistral shears (