華南南嶺地區早燕山期（侏羅紀）侵入岩定年學、全岩地球化學、鍶－釹同位素與磷灰石地球化學研究與地體構造意義

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

謝佩珊(Pei-Shan Hsieh)

指導教授 ： 陳正宏

國立臺灣大學/理學院/地質科學研究所/博士(2008年)

https://doi.org/10.6342/NTU.2008.03091

摘要

在華南廣泛出露的中生代侵入岩主要形成於三個時期：三疊紀（印支期）、侏羅紀（早燕山期）及白堊紀（晚燕山期）。岩性上來說，岩漿產物以酸性的花崗岩為主；在體積上，則以早燕山期岩漿活動最為強烈，產生的花崗岩占了中生代花崗岩近一半的出露面積。本研究針對南嶺地區的早燕山期侵入岩，分析鋯石鈾－鉛年齡、全岩與磷灰石地球化學及鍶－釹同位素組成，以討論早燕山期侵入岩的時空分佈與岩石成因。除此之外，還量測了南嶺地區的早燕山期花崗岩體型態，藉此瞭解花崗岩體與區域滑移斷層的關連，最後結合所有資料推論出華南中生代之地體構造演化。
南嶺早燕山期侵入岩基本上形成於三個時期：早期 (190-175 Ma)、主期 (165-150 Ma)與晚期 (145-135 Ma)。主要由高鉀鈣鹼性系列、在蛛網圖中表現出多變的大離子半徑元素含量、Nb, Sr, Ti, Zr和P虧損及地殼來源鍶－釹同位素值 (ISr=0.712~0.717; eNd(T)=-8.8~-11.6)的花崗岩基所組成。其他以小岩體方式侵入的岩類則是中鉀鈣鹼性系列輝長岩和鉀玄岩質正長岩類，蛛網圖中兩者無明顯Nb、Ta虧損現象，並有明顯地函來源貢獻的鍶－釹同位素組成 (ISr=0.704~0.708;eNd(T)=+3.0~-2.6)。利用磷灰石地球化學特徵判別S型與I型花崗岩，在本研究中成功地應用在華南三疊紀大容山和白堊紀福州－漳州花崗岩體上，然而侏羅紀南嶺花崗岩卻表現出介於兩者之間的中間特性。磷灰石的稀土元素分佈圖明顯與全岩的鋁飽和指數與釹同位素值有關，且南嶺花崗岩表現出不同於大容山、拉克蘭造山帶S型花崗岩磷灰石或福州－漳州岩體、拉克蘭造山帶I型花崗岩磷灰石之獨特的圖形。根據鍶－釹同位素結果推論出南嶺花崗岩具有兩個可能的地殼來源：主要為加里東期花崗岩質地殼與較次要的印支期花崗岩質地殼。
形成於早期的侵入岩（A型花崗岩、輝長岩）數量少且集中在贛南與十杭帶；早燕山主期以花崗岩為主的侵入岩大量產生，並成為南嶺地區的主體，同時也標誌了早燕山期岩漿事件的高峰，在短時間內產生廣泛的花崗岩岩漿活動，暗示著華南當時處於一個岩石圈極度張裂的狀態；早燕山晚期的侵入岩在數量上相對來說稀少，表示此時岩漿活動已經趨於和緩，晚期少數正長岩、雙模式火山岩與基性岩脈的出現標表示地函物質上湧侵入局部張裂地區，支援了這一個時期南嶺整體已趨於穩定，處於局部拉張的狀態。值得注意的是在整個早燕山期不同次期的岩漿活動中，特定地區（贛南、粵中、十杭帶）重複出現被認為是張裂環境下的岩漿產物，表示這些構造弱帶在整個早燕山期事件中持續提供管道，讓地函來源岩漿上升至地表。整體來說，原地地殼重熔似乎是最適當的模式來解釋大量產生的侏羅紀南嶺花崗岩。少數存在的鹼性岩、輝長岩或基性岩脈代表著地函物上湧侵入局部地表張裂處的現象。張裂或裂谷的環境造成來自於上部地函的基性岩漿（推測由軟流圈與富集岩石圈地函不同程度混合組成）底侵於地殼下方提供了大量的熱源讓地殼達到重熔的溫度，進而產生大量的地殼（加里東期和印支期地殼）來源的早燕山期花崗岩岩漿。呈現東西展布的南嶺花崗岩明顯受控於高錯移量的東西向滑移斷層，而構造的高錯移速度相信是造成正常對稱與同應力方向拉長型岩體的主因。
華夏陸塊上的印支期過鋁質花崗岩應屬於三疊紀時陸塊擠壓碰撞同造山期的產物，在印支期岩漿活動結束到早燕山期早期岩漿事件開始之間，存在約15 Ma的岩漿休止期，反應華南從印支期擠壓環境演變到早燕山期伸張環境的構造轉換，而這個轉變的控制原因，推測可能是碰撞加厚之華南陸塊岩石圈地函的拆層。岩石圈地函拆層讓地體環境從擠壓轉變為伸張，並重新活化了陸塊上已存在的古北東向及東西向滑移斷層系統，同時伴隨著有南北向拉張的產生，促使地函物質上湧。根據上述觀察與推論，本研究認為在侏羅紀太平洋板塊隱沒尚未影響華南陸塊的情形之下，早燕山期岩漿活動可合理解釋為印支期華南－印支半島陸陸碰撞造山運動的後造山地質事件 (post-orogenic event)。

關鍵字

中生代花崗岩 ； 鋯石鈾－鉛定年 ； 磷灰石地球化學 ； 南嶺 ； 華南

並列摘要

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 (

並列關鍵字

Mesozoic granitoids ； Nanling Mountains ； South China

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