Sr-Nd ISOTOPE AND GEOCHEMICAL EVIDENCE FOR THE ORIGIN OF CLINOPYROXENE, AMPHIBOLE, AND MICA MEGACRYSTS IN ALKALI BASALTIC ROCKS FROM SOUTHEASTERN CHINA AND PENGHU ISLANDS

華南及澎湖地區鹼性玄武岩內斜輝石、角閃石和雲母巨晶之成因：鍶－釹同位素和地球化學之證據

何恭算(KUNG-SUAN HO)

《Collection and Research》 12期 (1999/12) Pp. 1-31

https://doi.org/10.6693/CAR.1999.12.1

摘要

Clinopyroxene, amphibole, and mica megacrysts have been collected from late Cenozoic intraplate alkali basaltic rocks in SE China and the Penghu Islands and analyzed for both major and trace elements and Sr-Nd isotopes, Chemically clinopyroxene megacrysts can be divided into Al-augite, Fe-Na salite, and endiopside types. The Al-augites are relatively alumina- rich: the Fe-Na salites contain high ΣFeO, Na_2O, and incompatible elements contents; whereas the endiopsides have the highest MgO and Cr_2O_3 abundances. Seven amphibole megacrysts fall into two groups: kaersutite and pargasitelhastingsite. The former is higher in MgO, Al_2O_3, and TiO_2 and lower in Na_2O and ΣFeO contents than the latter. The Ti:Al ratio (Best, 1974) of the amphibole megacrysts is consistent with high-pressure derivation. Mica megacrysts belonging to titanphlogopites are characterized by high Mg values between 59.4 and 73.3. and have relatively low concentrations in 7 REEs from 0.55 to 1.28 ppm. In addition, they have a very pronounced positive Eu anomaly (δ Eu varying from +21.4 +85.7) in REE patterns. Partition coefficient (D) of clinopyroxene-melt for 24 trace elements have been computed using chemical data of Al-augite megacrysts and their host basalts analyzed by ICP-MS. In Al-augite megacrysts. Ba, Nb. Rb. Th, and La are highly incompatible (D ＜ 0.05), whereas Sc and V are compatible elements (D ＞ 1). Most of the partition coefficients obtained from this study are in accord with previously published experimental data (e.g., Skulski et al., 1994). From trace element abundances, Sr-Nd isotopic compositions, and partition coefficient data, most Al-augite and amphibole megacrysts are interpreted as cognates with the host magmas, whereas Fe-Na salite and mica megacrysts may represent fractional crystallization products of the evolved magma, and were brought up to the earth's surface by ascending basaltic magma at a later stage.

關鍵字

Megacrysts ； Late Cenozoic ； SE China and Penghu lslands

並列摘要

本文收集了華南和澎湖群島晚新生代板內鹼性玄武岩中之斜輝石、角閃石和雲母巨晶，並分析彼等之主要、微量元素和鍶－釹同位素。斜輝石巨晶可分爲鋁普通輝石、鐵－鈉次透輝石和鎂質透輝石三種類型。鋁普通輝石富鋁；鐵－鈉次透輝石含有高量的鐵、鈉及不共容元素；鎂質透輝石則具有最多的鎂及鉻含量。七件角閃石巨晶分屬鈦角閃石和鈉角閃石/富鐵鈉閃石，前者有較高的鎂、鋁、鈦及較低的鈉、鐵含量，而從鈦和鋁的原子比値(Best, 1974)可知，彼等均屬高壓源角閃石。雲母巨晶爲鈦質金雲母，具有高鎂値(59.4至73.3)和低稀土元素總量(0.55至1.28ppm)的特性，而其稀土元素分佈圖形有著非常明顯的銪正異常現象。本文利用感應耦合電漿質譜儀分析了鋁普通輝石和其玄武質母岩的微量元素含量，並計算出24個微量元素在斜輝石與熔體間的分配係數。對鋁普通輝石而言，鋇、鈮、銣、釷、鑭爲高度不共容元素(D＜0.05)；鈧、釩則爲共容性元素(D＞1)。經計算所得之分配係數，絶大多數數值與Skulski et al.(1994)等人所發表的實驗數據相當一致。綜合微量元素含量、鍶－釹同位素組成和分配係數資料顯示，大部分鋁普通輝石和角閃石巨晶與其母岩漿係屬同源；而鐵－鈉次透輝石和雲母巨晶則代表演化岩漿的結晶分化産物，並被後來上升的玄武質岩漿帶至地表。

並列關鍵字

巨晶 ； 晚新生代 ； 華南及澎湖群島