地球化学
地毬化學
지구화학
GEOCHIMICA
2015年
1期
27-42
,共16页
柏道远%钟响%贾朋远%熊雄%黄文义
柏道遠%鐘響%賈朋遠%熊雄%黃文義
백도원%종향%가붕원%웅웅%황문의
锆石SHRIMP U-Pb定年%S型花岗岩%北流运动%后碰撞构造环境%越城岭%湘西南
鋯石SHRIMP U-Pb定年%S型花崗巖%北流運動%後踫撞構造環境%越城嶺%湘西南
고석SHRIMP U-Pb정년%S형화강암%북류운동%후팽당구조배경%월성령%상서남
zircon SHRIMP U-Pb dating%S-type granitoid%Beiliu Movement%post-collisional tectonic setting%Yurchengling%southwestern Hunan Province
越城岭岩体位于南岭西段,主体为南部的加里东期花岗岩,北部为印支期花岗岩。加里东期花岗岩自早至晚依次为中细粒斑状黑云母二长花岗岩、细中粒斑状黑(二)云母二长花岗岩、细粒斑状黑(二)云母二长花岗岩和细粒黑(二)云母二长花岗岩。岩体东部和西部花岗岩分别具块状构造和片麻状构造。岩体西缘尚叠加了燕山期左行走滑-伸展型韧性剪切带。对中细粒斑状黑云母二长花岗岩和(糜棱岩化)细中粒斑状黑云母二长花岗岩各进行了1个样品的锆石SHRIMP U-Pb年龄测试,分别得到(436.6±4.8) Ma、(430.5±4.3) Ma的年龄值,反映花岗岩形成于早志留世晚期。岩石高硅、富铝、高钾、中碱, SiO2含量68.35%～78.10%,平均73.29%；Al2O3含量11.95%～15.55%,平均14.18%； K2O 含量4.12%～5.62%,平均4.95%；全碱(Na2O+K2O)含量为6.18%～8.30%,平均7.58%；K2O/Na2O值在1.36～2.82之间,平均1.94。ASI值1.04～1.66,平均1.23。总体属高钾钙碱性系列过铝质花岗岩类。大多数样品Ba、Nb、Sr、P、Ti表现为较强烈亏损, Rb、(Th+U+K)、(La+Ce)、Nd、(Zr+Hf+Sm)、(Y+Yb+Lu)等则相对富集；∑REE含量为50.43～328.81μg/g,平均173.39μg/g；δEu值0.21～0.68,平均为0.40；(La/Yb)N值为0.54～14.04,平均7.93；ISr值为0.71912和0.72415,εSr(t)值为208和279,εNd(t)值为–11.76～–7.80, t2DM为1.80～2.12 Ga。A/MF-C/MF图解显示源岩为泥质岩和碎屑岩。上述地球化学特征表明花岗岩为S型花岗岩,是陆壳碎屑岩石部分熔融的产物。花岗岩氧化物构造环境判别图解指示岩体形成于后碰撞构造环境。基于岩石成因、构造环境判别以及区域构造演化过程,推断加里东期越城岭花岗岩的具体形成机制为：奥陶纪末—志留纪初的北流运动导致地壳增厚、升温,早志留世中晚期在挤压减弱、应力松弛的后碰撞-减压构造环境下,中、上地壳酸性岩石发生部分熔融并向上侵位。
越城嶺巖體位于南嶺西段,主體為南部的加裏東期花崗巖,北部為印支期花崗巖。加裏東期花崗巖自早至晚依次為中細粒斑狀黑雲母二長花崗巖、細中粒斑狀黑(二)雲母二長花崗巖、細粒斑狀黑(二)雲母二長花崗巖和細粒黑(二)雲母二長花崗巖。巖體東部和西部花崗巖分彆具塊狀構造和片痳狀構造。巖體西緣尚疊加瞭燕山期左行走滑-伸展型韌性剪切帶。對中細粒斑狀黑雲母二長花崗巖和(糜稜巖化)細中粒斑狀黑雲母二長花崗巖各進行瞭1箇樣品的鋯石SHRIMP U-Pb年齡測試,分彆得到(436.6±4.8) Ma、(430.5±4.3) Ma的年齡值,反映花崗巖形成于早誌留世晚期。巖石高硅、富鋁、高鉀、中堿, SiO2含量68.35%～78.10%,平均73.29%；Al2O3含量11.95%～15.55%,平均14.18%； K2O 含量4.12%～5.62%,平均4.95%；全堿(Na2O+K2O)含量為6.18%～8.30%,平均7.58%；K2O/Na2O值在1.36～2.82之間,平均1.94。ASI值1.04～1.66,平均1.23。總體屬高鉀鈣堿性繫列過鋁質花崗巖類。大多數樣品Ba、Nb、Sr、P、Ti錶現為較彊烈虧損, Rb、(Th+U+K)、(La+Ce)、Nd、(Zr+Hf+Sm)、(Y+Yb+Lu)等則相對富集；∑REE含量為50.43～328.81μg/g,平均173.39μg/g；δEu值0.21～0.68,平均為0.40；(La/Yb)N值為0.54～14.04,平均7.93；ISr值為0.71912和0.72415,εSr(t)值為208和279,εNd(t)值為–11.76～–7.80, t2DM為1.80～2.12 Ga。A/MF-C/MF圖解顯示源巖為泥質巖和碎屑巖。上述地毬化學特徵錶明花崗巖為S型花崗巖,是陸殼碎屑巖石部分鎔融的產物。花崗巖氧化物構造環境判彆圖解指示巖體形成于後踫撞構造環境。基于巖石成因、構造環境判彆以及區域構造縯化過程,推斷加裏東期越城嶺花崗巖的具體形成機製為：奧陶紀末—誌留紀初的北流運動導緻地殼增厚、升溫,早誌留世中晚期在擠壓減弱、應力鬆弛的後踫撞-減壓構造環境下,中、上地殼痠性巖石髮生部分鎔融併嚮上侵位。
월성령암체위우남령서단,주체위남부적가리동기화강암,북부위인지기화강암。가리동기화강암자조지만의차위중세립반상흑운모이장화강암、세중립반상흑(이)운모이장화강암、세립반상흑(이)운모이장화강암화세립흑(이)운모이장화강암。암체동부화서부화강암분별구괴상구조화편마상구조。암체서연상첩가료연산기좌행주활-신전형인성전절대。대중세립반상흑운모이장화강암화(미릉암화)세중립반상흑운모이장화강암각진행료1개양품적고석SHRIMP U-Pb년령측시,분별득도(436.6±4.8) Ma、(430.5±4.3) Ma적년령치,반영화강암형성우조지류세만기。암석고규、부려、고갑、중감, SiO2함량68.35%～78.10%,평균73.29%；Al2O3함량11.95%～15.55%,평균14.18%； K2O 함량4.12%～5.62%,평균4.95%；전감(Na2O+K2O)함량위6.18%～8.30%,평균7.58%；K2O/Na2O치재1.36～2.82지간,평균1.94。ASI치1.04～1.66,평균1.23。총체속고갑개감성계렬과려질화강암류。대다수양품Ba、Nb、Sr、P、Ti표현위교강렬우손, Rb、(Th+U+K)、(La+Ce)、Nd、(Zr+Hf+Sm)、(Y+Yb+Lu)등칙상대부집；∑REE함량위50.43～328.81μg/g,평균173.39μg/g；δEu치0.21～0.68,평균위0.40；(La/Yb)N치위0.54～14.04,평균7.93；ISr치위0.71912화0.72415,εSr(t)치위208화279,εNd(t)치위–11.76～–7.80, t2DM위1.80～2.12 Ga。A/MF-C/MF도해현시원암위니질암화쇄설암。상술지구화학특정표명화강암위S형화강암,시륙각쇄설암석부분용융적산물。화강암양화물구조배경판별도해지시암체형성우후팽당구조배경。기우암석성인、구조배경판별이급구역구조연화과정,추단가리동기월성령화강암적구체형성궤제위：오도기말—지류기초적북류운동도치지각증후、승온,조지류세중만기재제압감약、응력송이적후팽당-감압구조배경하,중、상지각산성암석발생부분용융병향상침위。
Yuechengling pluton in the western segment of the Nanling Mountains is mainly composed of Caledonian and Indosinian granites in the southern and northern parts, respectively. Caledonian granites change with time from medium-fine-grained porphyritic biotite monzogranite, fine-medium-grained porphyritic two-mica monzogranite, fine-grained porphyritic two-mica monzogranite and fine-grained two-mica monzogranite. The Caledonian granites in the eastern part are characterized by massive and gneissic structures. Late Mesozoic extensional-type ductile shear zones occur in the western margin of this pluton. U-Pb zircon dating for a porphyritic biotite monzogranite sample and a mylonitic porphyritic biotite monzogranite sample yield weighted average ages of (436.6±4.8) Ma and (430.5±4.3) Ma, respectively, which indicate that the Yuechengling pluton was formed in late Early Silurian. The rocks are silicon-high (SiO2= 68.35%―78.10%, 73.29% on average), aluminium-rich (Al2O3= 11.95%―15.55%, 14.18% on average), potassium-high (K2O = 4.12%―5.62%, 4.95%on average) and alkali-moderate (Na2O+K2O = 6.18%―8.30%, 7.58% on average) with K2O/Na2O value of 1.36―2.82 (1.94 on average) and ASI value of 1.04―1.66 (1.23 on average), thus belong to high-K Calc-alkaline series perluminous granitoids. Most samples are depleted in Ba, Nb, Sr, P and Ti, and enriched in Rb, (Th+U+K), (La+Ce), Nd, (Zr+Hf+Sm) and (Y+Yb+Lu). The studied sampels have ∑REE values of 50.43―328.81 μg/g (173.39 μg/g on average), δEu values of 0.21―0.68 (0.40 on average), (La/Yb)N values of 0.54―14.04 (7.93 on average), ISr values of 0.71912 and 0.72415, εSr(t) values of 207 and 279,εNd(t) values of–11.76―–7.80 and t2DM ages of 1.80―2.12 Ga. The A/MF-C/MF diagram indicates that the Caledonian Yuechengling granites were form by partial melting of mudstones and clastic rocks. All above geochemical characteristics indicate that the granites are S-type granitoid. Multiple oxide-diagrams for discrimination of structural environment show that the granites were formed in post-collisional tectonic setting. According to petrogenesis, discrimination of structural environment and regional tectonic evolution, the formation mechanism of Caledonian Yuechengling granites is inferred as follows:the thickening of crust in Beiliu Movement (in the late Ordovician to the early Silurian) raised the crust temperature, granites formed through parting melting in post-collisional tectonic setting in middle-late Early Silurian.
