中國東部魯皖蘇地區新生代玄武岩之地球化學與岩石成因

Abstract

本研究分別在山東、安徽、江蘇等地區共採集79個玄武岩標本，並在江蘇省境內採集12個超基性捕獲岩（對組成礦物從事微探分析）進行全岩之主要、微量及稀土元素分析、氬-氬定年及鍶-釹同位素分析，探討各區玄武岩之時空變化、岩漿成因及分化、地函源區之特性、可能經歷之深部地質作用以及與地體構造之關係。 根據定年資料，魯皖蘇地區之火山活動可分為三期：早第三紀（古新世）、晚第三紀（漸新世-中新世）和第四紀（更新世）。研究區內火山岩隨著時間由老至新，岩性由矽質玄武岩逐漸轉變為橄欖玄武岩和鹼性玄武岩，晚第三紀（漸新世-中新世）為本區火山活動之過渡期，此時矽質玄武岩與橄欖玄武岩共存；另外本區火山岩之年齡由南（約13.7 Ma）往北（約0.82 Ma）稍有逐漸變年輕之趨勢。 江蘇、安徽和山東地區之玄武岩包括矽質玄武岩、橄欖玄武岩和鹼性玄武岩。山東地區玄武岩之組成礦物中橄欖石為主要之斑晶，其次為鈦輝石，橄欖石斑晶呈自形到半自形，鈦輝石斑晶則常具有環狀構造。安徽地區矽質玄武岩之斑晶主要為斜長石和斜輝石，基質則由斜長石、輝石、磁鐵礦和少量之玻璃質所組成；而鹼性玄武岩斑晶以橄欖石和鈦普通輝石為主。江蘇地區之鹼性玄武岩具微斑狀組織，斑晶以橄欖石為主，其次為含鈦普通輝石，而基質以橄欖石、斜輝石、斜長石和鈦鐵氧化物居多，部份岩樣之橄欖石具有環帶構造。橄欖玄武岩具班狀組織，斑晶以橄欖石和輝石為主，次為斜長石，基質主要由斜長石、輝石和磁鐵礦等礦物所組成。 山東省昌樂地區鹼性玄武岩之（La/Yb）N比值平均值為38.09、橄欖玄武岩之平均值為19.20；蓬萊地區鹼性玄武岩之平均值為52.85、橄欖玄武岩之平均值為42.73、矽質玄武岩之平均值為40.79，上述各類玄武岩（La/Yb）N比值隨著SiO2含量之減少有增大之現象，且蓬萊地區各類玄武岩輕稀土元素富集之程度明顯高於昌樂地區。安徽地區玄武岩稀土元素對SiO2含量作圖顯示除重稀土元素（Yb和Lu）外，整體而言鹼性玄武岩稀土元素之含量普遍高於矽質玄武岩，而矽質玄武岩共容元素（Ni、Co、Cr、Sc）之含量較鹼性玄武岩者稍高。江蘇地區玄武岩中不共容元素對MgO/ΣFeO作圖顯示各不共容元素均與MgO/ΣFeO呈現負變關係，表示在岩漿演化之早期此等不共容元素含量較低，但於晚期則有增高之現象。魯皖蘇地區玄武岩中不共容元素及稀土元素之分佈型態均與洋島玄武岩相似。 區內超基性捕獲岩之Sr87/Sr86（0.702907∼0.704349）和143Nd/144Nd比值（0.512659∼0.513371）變化較大，由虧損地函變至EMⅠ富化型地函，顯示區內之上部地函具有不均質性且部份地函可能均處於虧損狀態，但此地函可能在近期受交代換質作用之影響而富化。魯皖蘇地區尖晶石二輝橄欖岩之平衡溫壓為913℃∼1098℃，13kb∼24.2 kb，石榴子石二輝橄欖岩之平衡溫壓為1112℃∼1171℃，18.6 kb∼22.3 kb。此等捕獲岩之生成深度，應為地表下45∼83公里深處。研究區內上部地函之古地溫梯度線大致介於裂谷-洋脊與大洋地溫線之間且區內尖晶石二輝橄欖岩之落點相當接近大洋地溫線，顯示魯皖蘇地區新生代上部地函應處於「較熱」之狀態，因此區內可能有軟流圈上湧之現象，當軟流圈之物質上升至岩石圈底部時由於壓力減小，可在岩石圈與軟流圈交界處產生不同比 綜合區內玄武岩與超基性捕獲岩之研究資料，筆者認為魯皖蘇地區之岩石圈地函可能已發生過部份熔融，使其處於較虧損之狀態，但流體或熔體之交代換質作用事件可能於近期產生，造成此岩石圈地函化學組成呈現不均質之現象。由於地函對流或物質再循環等地質作用使該區地函受到富含CO2和H2O流體之交代換質作用，局部的富化了區內之岩石圈地函。在新生代時期，拉張作用可能使軟流圈產生上湧或貫入加熱了岩石圈地函之底部，使該處發生不同比例之部分熔融，部份上湧之物質亦可能與岩石圈地函產生不同比例之混合，由於加熱與解壓作用使上述之物質產生不同比例之部分熔融可形成境內各類之玄武質母岩漿。

Abstract Seventy nine Cenozoic basaltic rocks from Shandong, Anhui and Jiangsu provinces and twelve ultramafic xenoliths from Jiangsu province have been analyzed for major and trace element contents and Sr-Nd isotopic compositions as well as 40Ar-39Ar dating, in order to (1) investigate the temporal and spacial variation of these basaltic rocks, and (2) discuss the origin and fractionation of the parental magma, and (3) identify the geological processes beneath the study area, and (4) evaluate the relationship between the basaltic rocks and geological setting. Based on 40Ar-39Ar and K-Ar dating, the basaltic volcanism in the study area may be divided into three eruptive episodes: Paleocene, Oligocene to Miocene and Pleistocene. The rock type of analyzed basaltic rocks vary from olivine tholeiite to alkali basalts with time. The eruptive episode of Oligocene to Miocene is a transitional one and tholeiite coexists with olivine tholeiite during that episode. In addition, the age of the analyzed basaltic rocks tends to become younger from south (about 13.7 Ma) to north (about 0.82 Ma). The common rock types of basaltic rocks from Shandong, Anhui and Jiangsu provinces include quartz tholeiite, olivine tholeiite and alkali basalt. The phenocrysts of basalts from Shandong province consist mainly of olivine (euhedral to subhedral) with minor amount of zoned titanaugite. The phenocrysts of tholeiite from Anhui province consist predominantly of plagioclase and clinopyroxene and the groundmass is mainly composed of plagioclase, pyroxene and magnetite. The major phenocrysts are olivine and titanaugite in the alkali basalt. The phenocrysts of alkali basalt from Jiangsu provinces consist mainly of olivine with minor titanaugite and the groundmass is mainly composed of olivine, clinopyroxene, plagioclase and Ti-Fe oxide minerals. The olivine tholeiite are mostly porphyritic with olivine and pyroxene as the major phenocrysts. The groundmass is mainly composed of pyroxene, plagioclase and magnetite. The average (La/Yb)N ratios of basaltic rocks from Changlo area are 38.09 and 19.20 for alkali basalt and olivine tholeiite respectively. The average (La/Yb)N ratios of basaltic rocks from Penglai area are 52.85, 42.73 and 40.79 for alkali basalt, olivine tholeiite and tholeiite respectively. The (La/Yb)N ratios of basalts from Shandong provinces increase with decreasing SiO2 contents and LREEs in basalts from Penglai area are higher than those from Changlo area. The rare earth element contents in alkali basalts are generally higher than those in tholeiites expect for Yb and Lu. In addition, the compatible elements (Ni, Co, Cr, Sc) contents in tholeiites are slightly higher than those in alkali basalts. In Jiangsu basaltic rocks the incompatible elements vs. MgO/ΣFeO plots show negative trends indicating that these elements are relatively lower in the early stage of magmatic evolution. The primitive mantle-normalized incompatible elements and chondrite-normalized REE patterns of basaltic rocks of this study are similar to those of OIB. The 87Sr/86Sr (0.702907~0.704349) and 143Nd/144Nd (0.512659~0.513371) ratios show wider variations, ranging from depleted mantle to EMI-type mantle, indicating upper mantle heterogeneity and some parts of the upper mantle may have undergone the metasomatism event. Calculation based on olivine-spinel geothermometries and orthopyroxene-clinopyroxene geothermo- barometry indicate that the equilibrium conditions of the spinel lherzolite xenoliths and garnet lherzolite xenoliths in the study area are: T= 913-1098℃ , P= 13-24.2kb and T= 1112 ℃-1171℃, P= 18.6-22.3 kb corresponding to depths of 45-83 km. The geothermometer suggests that the geothermal gradient of the upper mantle beneath the study area is approximately between MORB and oceanic geotherm indicating that the mantle may be under a “ hotter” condition. We suggest that lithospheric mantle thinning accompanied by asthenosphere upwelling (or diaprism) caused decompressed partial melting in the study area. Based on geochemical data of the basalts and ultramafic xenoliths we suggest that the lithospheric mantle may have undergone partial melting which caused the mantle depletion. Part of the upper mantle in the study area may have experienced fluid (enriched in CO2 and H2O) metasomatism resulted in chemical heterogeneity. During Cenozoic era, the extension process may cause asthenosphere upwelling (or diaprism) to heat the bottom of the lithospheric mantle which gave rise to different degree of partial melting and parts of the melts may move upward to mix with the lithospheric mantle and produce different parental magmas in the study area.