Petrogenesis of ultramafic and mafic xenoliths from Mesozoic basanites in southern Sweden: constraints from mineral chemistry
Petrogenesis of ultramafic and mafic xenoliths from Mesozoic basanites in southern Sweden: constraints from mineral chemistry
复制标题
瑞典南部中生代硅岩中超镁铁质和镁铁质捕虏体的岩石成因：矿物化学的限制
DOI：
10.1007/s00531-006-0116-4
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发表时间：
2007
影响因子：
2.3
通讯作者：
L. Johansson
中科院分区：
文献类型：
--
作者：
Tatjana Rehfeldt;K. Obst;L. Johansson
侏罗纪碧玄岩颈发生在斯堪尼亚的两个主要断裂带的交界处，包含超镁铁质（橄榄岩，辉石岩）和镁铁质捕虏体，这共同表明该地区下的上地幔和下地壳组合的多样性。橄榄岩可分为二辉橄榄岩、纯橄榄岩和方辉橄榄岩。大部分二辉橄榄岩为碎斑岩，含有斜方辉石和橄榄石碎斑岩。它们由富镁硅酸盐（Mg# = Mg/（Mg + Fetot）× 100; 88-94）和蠕虫状尖晶石组成。计算出的碎斑二辉橄榄岩的平衡温度（975- 1，007 °C）低于等粒二辉橄榄岩（1，079 °C），表明其起源于上地幔的不同部分。根据尖晶石组成，尖晶石代表8-13%部分熔融的残余物。它们在结构、矿物学和主要元素组成上与中欧新生代火山岩区的地幔捕虏体相似。纯橄榄岩和方辉橄榄岩是等粒的，只有轻微变形。硅酸盐矿物的Mg#（83-92）与尖晶石相近，但不含原生尖晶石。纯橄榄岩和哈氏橄榄岩捕虏体中的再吸收斑块可能是改变上地幔成分的交代作用的残余。辉石是粗糙的，未变形的，并且具有硅酸盐矿物，其Mg#比橄榄岩部分地低（70-91）。辉石氧化物是多镁尖晶石。根据二辉石测温法，辉石岩的平衡温度范围很大（919- 1，280 °C）。相比之下，镁铁质捕虏体主要是富含辉石和斜长石的层状辉长岩，其平衡温度范围很窄（828-890°C）。这些温度范围，再加上地球化学证据，表明辉石岩和辉长岩捕虏体代表芬诺斯堪的纳维亚地壳内的镁铁质侵入体。
Jurassic basanite necks occurring at the junction of two major fault zones in Scania contain ultramafic (peridotites, pyroxenites) and mafic xenoliths, which together indicate a diversity of upper mantle and lower crustal assemblages beneath this region. The peridotites can be subdivided into lherzolites, dunites and harzburgites. Most lherzolites are porphyroclastic, containing orthopyroxene and olivine porphyroclasts. They consist of Mg-rich silicates (Mg# = Mg/(Mg + Fetot) × 100; 88–94) and vermicular spinel. Calculated equilibration temperatures are lower in porphyroclastic lherzolites (975–1,007°C) than in equigranular lherzolite (1,079°C), indicating an origin from different parts of the upper mantle. According to the spinel composition the lherzolites represent residues of 8–13% fractional melting. They are similar in texture, mineralogy and major element composition to mantle xenoliths from Cenozoic Central European volcanic fields. Dunitic and harzburgitic peridotites are equigranular and only slightly deformed. Silicate minerals have lower to similar Mg# (83–92) as lherzolites and lack primary spinel. Resorbed patches in dunite and harzburgite xenoliths might be the remnants of metasomatic processes that changed the upper mantle composition. Pyroxenites are coarse, undeformed and have silicate minerals with partly lower Mg# than peridotites (70–91). Pyroxenitic oxides are pleonaste spinels. According to two-pyroxene thermometry pyroxenites show a large range of equilibration temperatures (919–1,280°C). In contrast, mafic xenoliths, which are mostly layered gabbronorites with pyroxene- and plagioclase-rich layers, have a narrow range of equilibration temperatures (828–890°C). These temperature ranges, together with geochemical evidence, indicate that pyroxenites and gabbroic xenoliths represent mafic intrusions within the Fennoscandian crust.