華南陸塊中部早古生代及早中生代花崗岩類年代及地球化學研究

Abstract

在華南陸塊中部（武夷－雲開造山帶以西、雪峰山造山帶以東）廣泛出露早古生代與早中生代侵入岩，岩性多為中性及酸性花崗岩類。華南陸塊雖由西北的揚子地塊與東南的華夏地塊所拼合而成，然而聚合時間與機制仍存在不同見解，特別在華南中部湖南一帶的拼合位置迄今未明。本研究針對本區兩時代花崗岩類分析鋯石鈾－鉛年齡及全岩地球化學，以討論早古生代與早中生代侵入岩的岩石成因，並據以檢視可能的揚子與華夏地塊交界及其拼合史。 本區早古生代花崗岩類岩體侵入時代可分為兩期。第一期之年齡範圍在444~423 Ma，峰值為437 Ma，形成華夏地塊的萬洋山、北諸廣山、彭公廟、丰頂山以及揚子地塊的板杉鋪、宏廈橋等大量I型花崗岩岩體。另也形成局部S型花崗岩於上述萬洋山、彭公廟兩岩體以及介於S型－I型間的過渡型花崗岩於揚子地塊的苗兒山、越城嶺、海洋山等花崗閃長岩及花崗岩岩體。第二期之年齡範圍在418~400 Ma，峰值為403 Ma，形成少量I型花崗岩疊加於丰頂山岩體北側、彭公廟岩體北側、北諸廣岩體與板杉鋪岩體；並有少量仍具過渡型特徵花崗岩疊加於前期苗兒山、越城嶺岩體。兩期I型花崗岩在各岩體中主要岩漿演化機制為由花崗閃長岩至花崗岩質岩漿的結晶分化，不同岩體之間則有地殼混染的程度差異。 本區早中生代花崗岩類岩體侵入時代亦分為兩期。第一期之年齡範圍為232~217 Ma，峰值為222 Ma，不僅產生I型花崗岩，包含揚子地塊的溈山、紫雲山、關帝廟等花崗岩體及白馬山、歇馬等花崗閃長岩岩體，而且產生大量S型（含過渡型）花崗岩，包含揚子地塊的塔山、陽明山、五峰仙花崗岩岩體，以及華夏地塊的丫江橋、鄧阜仙、小溪洞、南諸廣等花崗岩岩體。第二期之年齡範圍為214~206 Ma，峰值為212 Ma，再次形成I型花崗岩質岩漿疊加於前期產生的溈山、紫雲山、白馬山、歇馬岩體，並形成過渡型岩漿侵入或疊加於早古生代的苗兒山、越城嶺岩體。各岩體中的化學組成主要受到結晶分化所控制，不同岩體之間則有地殼混染的程度差異。 揚子與華夏地塊的交界一般認為是江山－紹興斷裂帶的延伸連接至郴州－臨武斷裂帶，早古生代I型花崗岩廣泛分布於其兩側。地體構造投圖顯示兩期I型花崗岩均屬於火山弧環境，第一期S型花崗岩屬於後碰撞環境，第二期過渡型花崗岩則屬於板內環境。根據此結果，推論早古生代時期揚子地塊與華夏地塊已拼合，可能受到類似地函熱柱的湧昇熱源影響形成陸內造山，在兩個地塊上均形成帶有地函來源特徵的I型花崗岩及上部地殼熔融而成的S型及過渡型花崗岩。 早中生代第一期的I型與S型花崗岩岩體呈區域性分布，但分布界線並未沿著江山－紹興斷裂帶的延伸及郴州－臨武斷裂帶，而是跨越揚子地塊呈現東北東－西南西走向，地體構造投圖顯示西北的I型花崗岩屬於火山弧構造環境，東南的S型花崗岩則屬於同碰撞至板內環境。早中生代第二期，I型與S型花崗岩岩體均分布於安化－羅城斷裂帶東側，北邊的I型延續第一期帶有火山弧構造環境特徵，南邊的S型則延續早古生代第二期，仍具有板內構造環境特徵。早中生代花崗岩類S型與I型的分布，顯示花崗岩的形成並非如過去認知受控於西南邊印支造山運動（中南半島與華南陸塊碰撞）或北邊華北陸塊向華南碰撞拼合，而可能是受到華南西南外海縫合構造的遠程效應所致。

There are widespread Early Paleozoic and Early Mesozoic intrusives, ranging from granodiorite to silicic granite, in the South China Block (SCB). As the SCB is composed of the Yangtze and Cathaysia sub-blocks, however, different views on the amalgamation age and mechanism, and even the boundary of these two sub-blocks are under debate. In this study, the zircon U-Pb dating and whole-rock geochemistry of granitoids occurring at these two eras in the area bounded by the west of Wuyi-Yunkai orogen and the east of Xuefengshan orogen (central SCB) are analyzed to better understand the petrogenesis and interaction of these two sub-blocks. Two stages of Early Paleozoic granitoids in central SCB can be divided. For the first stage, Wanyangshan, North Zhuguangshan, Penggongmiao, Fengdinshan granitic plutons in the Cathaysia sub-block emplaced at 449-431 Ma, Miaoershan, Yuechengling, Haiyangshan granodioritic to granitic plutons and Banshanpu, Hongxiaqiao granitic plutons in the Yangtze sub-block emplaced at 435-424 Ma. The whole-rock geochemical results, e.g. the ANK versus ACNK diagrams, show that most plutons in the Cathaysia and Yangtze sub-blocks belong to I-type granitoids, while Wanyangshan and Penggongmiao plutons in the Cathaysia sub-block are S-type granitoids, Miaoershan, Yuechengling and Haiyangshan plutons are transitional-type between S- and I-type. For the second stage, northern Penggongmiao and Fengdinshan plutons in the Cathaysia sub-block had emplaced with granites again at 405-402 Ma. However, Banshanpu, Miaoershan, Yuechengling plutons in the Yangtze sub-block were newly formed at 418-400 Ma. Except Miaoershan and Yuechengling plutons are transitional-type, other plutons in the second stage are I-type granitoids. Fractional crystallization is the major mechanism responsible for the evolution of granodioritic to granitic magmas. Also two stages of Early Mesozoic granitoids are recognized in central SCB. For the first stage, Baimashan, Xiema granodioritic plutons and Weishan, Ziyunshan, Guandimiao granitic plutons in the Yangtze sub-block emplaced at 243-234 Ma. On the other hand, Tashan, Yangmingshan, Wufengxian granitic plutons in the Yangtze and Xiaoxidong alkali granitic pluton, Yajiangqiao, Dengfuxian, South Zhuguangshan granitic plutons in the Cathaysia sub-blocks emplaced at 239-223 Ma. The whole-rock geochemical results show that plutons in the former group are I-type granitoids, whereas the latter group are S-type granitoids. For the second stage, Baimashan, Weishan, Xiema, Ziyunshan and Miaoershan, Yuechengling plutons in the Yangtze sub-block emplaced again at 222-208 Ma. Except Miaoershan, Yuechengling plutons are transitional-type, other granitoids in the second stage are I-type. The Early Paleozoic first stage I-type granitoids are widespread in two sides of the boundary connecting the extension of Jiangshan-Shaoxing Fault and Chenzhou-Linwu Fault. Tectonic discrimination diagrams show an arc-continent collision environment of I-type granitoids, but a post-collisional environment of S-type granitoids for the first stage magmatism. For the second stage, tectonic discrimination diagrams show a volcanic environment of I-type granitoids. An intracontinental plume-like upwelling producing the two pulses of granitoids in the central SCB is the favorite model. This implies that Yangtze and Cathaysia sub-blocks might have been amalgamated already before the early Paleozoic. In the Early Mesozoic, an ENE-WSW trending I-S deviding line of first stage rocks in the Yangtze sub-blocks is constructed, which is different from the extension of Jiangshan-Shaoxing Fault Zone and Chenzhou-Linwu Fault. For the second stage, both I-type and S-type granitoids are closed to the Anhua-Luocheng Fault. Discrimination diagrams for the tectonic interpretation shows a syn-collision environment at southeast, while an arc-continent environment at northwest. Although generation of the Early Mesozoic granitoids in this area were commonly related to the Indosinian orogeny (collision between Indochina and South China Blocks), coeval I- and S-type granitoids were suggested to reflect an intracratonic orogeny likely affected by the hypothetic Triassic suture along offshore of the SCB as advocated by Faure et al., 2016.