大地构造与成矿学
大地構造與成礦學
대지구조여성광학
GETECTONICA ET METALLOGENIA
2013年
4期
698-720
,共23页
关义立%袁超%龙晓平%王毓婧%张运迎%黄宗莹
關義立%袁超%龍曉平%王毓婧%張運迎%黃宗瑩
관의립%원초%룡효평%왕육청%장운영%황종형
花岗岩%陆内造山带%下地壳加厚%华南地块%软流圈上涌
花崗巖%陸內造山帶%下地殼加厚%華南地塊%軟流圈上湧
화강암%륙내조산대%하지각가후%화남지괴%연류권상용
granitoids%intracontinental orogenic belt%thickened lower crust%South China Block%upwelling of asthenosphere
华南地块东部的早古生代造山带是特殊的陆内造山带(主要分布于安化-罗城断裂和政和-大埔断裂之间),以广泛的强烈变质、变形作用和大量分布的花岗质岩为特征。华南地块东部地区的早古生代花岗岩以过铝质S型花岗岩为主, I型花岗岩分布少且研究程度相对较低。目前,对于华南大面积花岗岩的成因一直存在较大争议,是否有地幔物质的参与也并不明确。对扬子板块东南缘板杉铺和宏夏桥两个I型花岗闪长岩岩体的锆石U-Pb定年结果显示,板杉铺和宏夏桥岩体分别形成于432.4±2.6 Ma 和434.3±2.7 Ma。系统的地球化学分析显示,板杉铺岩体富 K2O(2.71%~3.46%)和Al2O3(15.17%~15.59%),具有高 Sr(344.8×10-6~466.6×10-6)、Sr/Y 比值(34~47)和低 Y(8.51×10-6~10.72×10-6),具有埃达克质岩石特征。其低εNd(t)(-7.5~-6.9)和高的87Sr/86Sri(0.7130~0.7169),以及较老的tDM2=1729~1776 Ma,暗示其来源于加厚的下地壳中古元古代物质的部分熔融。相比板杉铺岩体,宏夏桥岩体具有较低的 SiO2(64.40%~65.42%)含量,较高的TiO2(0.58%~0.63%)、Fe2O3(4.48%~4.79%)、CaO(3.34%~4.39%)和 MgO(2.55%~3.06%),以及较高的 Y 含量(14.56×10-6~17.82×10-6)和较低的 Sr/Y(20~24)比值。但其与板杉铺岩体相似的 Sr-Nd 同位素特征(87Sr/86Sri=0.7142~0.7149,εNd(t)=-7.7~-7.3),说明宏夏桥岩体具有与板杉铺岩体相似的物质源区。由于高钾钙碱性I型花岗质岩的形成需要有外部热源提供才能使其源区发生部分熔融,因而该地区陆内造山峰期(444 Ma)之后的垮塌所导致的高温软流圈上涌,可能为这些花岗质岩石的形成提供了热源。但是这些岩体富集的 Sr-Nd 同位素组成显示,软流圈来源的地幔物质并没有直接参与花岗质岩的形成。结合前人的研究成果,造山后伸展环境下的软流圈上涌可能是形成华南特殊的早古生代陆内造山带的一种重要机制,是形成华南有别于经典陆内造山带的大量早古生代花岗岩的主要原因。
華南地塊東部的早古生代造山帶是特殊的陸內造山帶(主要分佈于安化-囉城斷裂和政和-大埔斷裂之間),以廣汎的彊烈變質、變形作用和大量分佈的花崗質巖為特徵。華南地塊東部地區的早古生代花崗巖以過鋁質S型花崗巖為主, I型花崗巖分佈少且研究程度相對較低。目前,對于華南大麵積花崗巖的成因一直存在較大爭議,是否有地幔物質的參與也併不明確。對颺子闆塊東南緣闆杉鋪和宏夏橋兩箇I型花崗閃長巖巖體的鋯石U-Pb定年結果顯示,闆杉鋪和宏夏橋巖體分彆形成于432.4±2.6 Ma 和434.3±2.7 Ma。繫統的地毬化學分析顯示,闆杉鋪巖體富 K2O(2.71%~3.46%)和Al2O3(15.17%~15.59%),具有高 Sr(344.8×10-6~466.6×10-6)、Sr/Y 比值(34~47)和低 Y(8.51×10-6~10.72×10-6),具有埃達剋質巖石特徵。其低εNd(t)(-7.5~-6.9)和高的87Sr/86Sri(0.7130~0.7169),以及較老的tDM2=1729~1776 Ma,暗示其來源于加厚的下地殼中古元古代物質的部分鎔融。相比闆杉鋪巖體,宏夏橋巖體具有較低的 SiO2(64.40%~65.42%)含量,較高的TiO2(0.58%~0.63%)、Fe2O3(4.48%~4.79%)、CaO(3.34%~4.39%)和 MgO(2.55%~3.06%),以及較高的 Y 含量(14.56×10-6~17.82×10-6)和較低的 Sr/Y(20~24)比值。但其與闆杉鋪巖體相似的 Sr-Nd 同位素特徵(87Sr/86Sri=0.7142~0.7149,εNd(t)=-7.7~-7.3),說明宏夏橋巖體具有與闆杉鋪巖體相似的物質源區。由于高鉀鈣堿性I型花崗質巖的形成需要有外部熱源提供纔能使其源區髮生部分鎔融,因而該地區陸內造山峰期(444 Ma)之後的垮塌所導緻的高溫軟流圈上湧,可能為這些花崗質巖石的形成提供瞭熱源。但是這些巖體富集的 Sr-Nd 同位素組成顯示,軟流圈來源的地幔物質併沒有直接參與花崗質巖的形成。結閤前人的研究成果,造山後伸展環境下的軟流圈上湧可能是形成華南特殊的早古生代陸內造山帶的一種重要機製,是形成華南有彆于經典陸內造山帶的大量早古生代花崗巖的主要原因。
화남지괴동부적조고생대조산대시특수적륙내조산대(주요분포우안화-라성단렬화정화-대포단렬지간),이엄범적강렬변질、변형작용화대량분포적화강질암위특정。화남지괴동부지구적조고생대화강암이과려질S형화강암위주, I형화강암분포소차연구정도상대교저。목전,대우화남대면적화강암적성인일직존재교대쟁의,시부유지만물질적삼여야병불명학。대양자판괴동남연판삼포화굉하교량개I형화강섬장암암체적고석U-Pb정년결과현시,판삼포화굉하교암체분별형성우432.4±2.6 Ma 화434.3±2.7 Ma。계통적지구화학분석현시,판삼포암체부 K2O(2.71%~3.46%)화Al2O3(15.17%~15.59%),구유고 Sr(344.8×10-6~466.6×10-6)、Sr/Y 비치(34~47)화저 Y(8.51×10-6~10.72×10-6),구유애체극질암석특정。기저εNd(t)(-7.5~-6.9)화고적87Sr/86Sri(0.7130~0.7169),이급교로적tDM2=1729~1776 Ma,암시기래원우가후적하지각중고원고대물질적부분용융。상비판삼포암체,굉하교암체구유교저적 SiO2(64.40%~65.42%)함량,교고적TiO2(0.58%~0.63%)、Fe2O3(4.48%~4.79%)、CaO(3.34%~4.39%)화 MgO(2.55%~3.06%),이급교고적 Y 함량(14.56×10-6~17.82×10-6)화교저적 Sr/Y(20~24)비치。단기여판삼포암체상사적 Sr-Nd 동위소특정(87Sr/86Sri=0.7142~0.7149,εNd(t)=-7.7~-7.3),설명굉하교암체구유여판삼포암체상사적물질원구。유우고갑개감성I형화강질암적형성수요유외부열원제공재능사기원구발생부분용융,인이해지구륙내조산봉기(444 Ma)지후적과탑소도치적고온연류권상용,가능위저사화강질암석적형성제공료열원。단시저사암체부집적 Sr-Nd 동위소조성현시,연류권래원적지만물질병몰유직접삼여화강질암적형성。결합전인적연구성과,조산후신전배경하적연류권상용가능시형성화남특수적조고생대륙내조산대적일충중요궤제,시형성화남유별우경전륙내조산대적대량조고생대화강암적주요원인。
The early Paleozoic intracontinental orogenic belt in the eastern South China Block is characterized by not only intensive deformation and metamorphism, but also extensive granitic magmatism. Granites in the eastern South China Block are predominately peraluminous S-type granites, and I-type granites are rare. Uncertainties remain for the petrogenesis of the I-type granitic intrusions in the area, especially the role of the mantle. Two representative granodioritic plutons in southeastern Yangtze Block, namely, the Banshanpu and Hongxiaqiao plutons were selected for LA-ICP-MS zircon dating and geochemistry analysis. The results indicated that the Banshanpu and Hongxiaqiao plutons were emplaced at 432.4±2.6 Ma and 434.3±2.7 Ma respectively, right on the peak time (434 Ma) of the early Paleozoic magmatism in South China Block. The Banshanpu pluton has high K2O (2.71%~3.46 %) and Al2O3 (15.17%~15.59%) contents with high Sr/Y(34~47) ratios, similar to adakitic rocks. The low εNd(t) (-7.5~-6.9) values, high 87Sr/86Sri (0.7130~0.7169) ratio and old tDM2 (1729~1776 Ma) indicate an origination of thickened Paleoproterozoic lower crust. In contrasting, the Hongxiaqiao pluton has relatively low SiO2 (64.40%~65.42%) content, higher TiO2 (0.58%~0.63%), Fe2O3(4.48%~4.79%), CaO (3.34%~4.39%), MgO (2.55%~3.06%) and Y (14.56×10-6~17.82×10-6) contents and lower Sr/Y (20~24) ratios. Sr-Nd isotopes (87Sr/86Sri=0.7142~0.7149,εNd(t)=-7.7~-7.3) of the Hongxiaqiao pluton are similar to those of the Banshanpu pluton, suggesting a similar source(s). Unlike S-type granites, petrogenesis of I-type granite requires extra heat to fuse the lower crust source. The post-orogenic collapse after intracontinental peak-metamorphism (444 Ma) led to the upwelling of the asthenosphere, which provided enough heat for partial melting of the source rocks, but the asthenosphere did not provide material for the granitic rocks. It is most likely that the upwelling of asthenosphere created the early Paleozoic intracontinental orogenic belt in Southern China Block.
