Geochemistry and origin of the basal lherzolites from the northern Oman ophiolite (northern Fizh block)
Geochemistry and origin of the basal lherzolites from the northern Oman ophiolite (northern Fizh block)
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DOI：
10.1029/2001gc000232
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发表时间：
2003-02
期刊：
影响因子：
3.7
通讯作者：
E. Takazawa;Toshie Okayasu;K. Satoh
中科院分区：
文献类型：
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作者：
E. Takazawa;Toshie Okayasu;K. Satoh
阿曼北部蛇绿岩中橄榄岩和方辉橄榄岩的全岩及矿物中主量和微量元素的丰度被用于了解造成大洋岩石圈地幔成分变化的地幔过程。详细的填图显示，橄榄岩出现在菲兹地块北部地幔剖面底部附近。地球化学分析确定了两种类型的底部橄榄岩。第一种类型（I型橄榄岩）呈现出残斑结构微观构造，零星出现在底部糜棱岩带中。全岩和单斜辉石在不相容元素如钠、钛、锆以及稀土元素（REE）方面高度亏损。I型橄榄岩的球粒陨石标准化模式从重稀土（HREE）到轻稀土（LREE）呈现陡坡，这归因于从含有少量石榴石的源区抽取了高达12 - 18%的熔体。球粒陨石标准化模式相对于部分熔融残余物所预期的模式在轻稀土方面有轻微富集，从而表明在低熔体/岩石比的情况下与一种富轻稀土的熔体或流体发生了反应。第二种类型（II型橄榄岩）呈现出糜棱结构微观构造，且仅出现在地幔剖面与变质基底的接触部位。全岩和单斜辉石中不相容元素的丰度大于I型橄榄岩，并且II型橄榄岩中的单斜辉石具有较高的氧化钠含量（>1 wt.%）。大致来说，单斜辉石和全岩的高钠含量以及轻稀土亏损、球粒陨石标准化的全岩稀土模式与II型橄榄岩在相对高压（>2 GPa）下与大洋中脊玄武岩（MORB）类型的熔体处于平衡状态是一致的。然而，中重稀土的球粒陨石标准化模式的平坦度与残余石榴石橄榄岩不一致。II型橄榄岩的特征可以通过一种混合过程更好地解释，即残余橄榄岩通过加入一种贫轻稀土的熔体而重新富集。菲兹地块北部底部逆冲断层附近较大的成分梯度可能记录了一种瞬态，即由于地幔温度降低和上涌速率减小，部分熔融程度逐渐降低。这种情况与Nicolas等人[2000]提出的与菲兹地块西部转换带相关的推断的衰退洋脊是一致的。在阿曼蛇绿岩的拆离阶段，少量上升的熔体可能在地幔剖面底部附近结晶，从而形成II型橄榄岩。菲兹地块北部的底部橄榄岩及其空间化学变化可能为理解快速扩张洋脊的洋脊分段和拆离过程提供关键线索。
Abundances of major and trace elements in whole rocks and minerals in lherzolites and harzburgites from the northern Oman ophiolite are used to understand the mantle processes creating compositional variation in oceanic lithospheric mantle. Detailed mapping shows that lherzolites occur near the base of a mantle section in the northern Fizh block. Geochemical analyses identify two types of basal lherzolite. The first type (Type I lherzolite) displays porphyroclastic microstructure and occurs sporadically in the basal mylonite zone. Whole rock and clinopyroxene are highly depleted in incompatible elements such as Na, Ti, Zr, and rare earth elements (REE). The chondrite‐normalized patterns of Type I lherzolites show steep slopes from heavy REE (HREE) to light REE (LREE) that are ascribed to melt extraction, up to 12–18%, from a source containing a small amount of garnet. The chondrite‐normalized patterns have slight enrichment in LREE relative to the patterns expected for residues of partial melting thereby indicating reaction with a LREE‐enriched melt or fluid at a low melt/rock ratio. The second type (Type II lherzolite) shows mylonitic microstructure and only occurs at the contact between the mantle section and the metamorphic sole. Abundances of incompatible elements in whole rocks and clinopyroxenes are greater than those of Type I lherzolites, and clinopyroxenes in Type II lherzolites have high Na2O contents (>1 wt.%). To a first approximation, the high Na content of clinopyroxenes and whole rocks and the LREE‐depleted, chondrite‐normalized whole rock REE patterns are consistent with Type II lherzolite being in equilibrium with a mid‐ocean ridge basalt (MORB)‐type melt at relatively high pressure (>2 GPa). However, the flatness of chondrite‐normalized patterns for middle and heavy REE are inconsistent with residual garnet peridotite. The characteristics of Type II lherzolites are better explained by a mixing process in which residual peridotite was refertilized by addition of a LREE‐depleted melt. The large compositional gradient near the basal thrust in the northern Fizh block may have recorded a transient state in which the degree of partial melting was progressively decreased as a result of reducing mantle temperature and upwelling rate. This scenario is consistent with the inferred failing ridge associated with a transform zone in the western side of the northern Fizh block proposed by Nicolas et al. [2000]. In the detachment stage of the Oman ophiolite, a small amount of ascending melt may have crystallized near the basal part of mantle section thereby forming Type II lherzolites. Basal lherzolites and their spatial chemical variations in the northern Fizh block may provide a key for understanding the processes of ridge segmentation and detachment at fast spreading ridges.