Annotated record of the detailed examination of Mn deposits from the QUEBRADA (1969) and SIQUEIROS (1974) expeditions around the East Pacific Rise@en

Basalt samples obtained from the Siqueiros transform fault/fracture zone and the adjacent East Pacific Rise are mostly very fresh oceanic tholeiite and fractionated oceanic tholeiite with Fe+3/ Fe+2 ? 0.25; however, alkali basalts occur in the area as well. The rocks of the tholeiitic suite are ol + pl phyric and ol + pl + cpx phyric basalts, while the alkali basalts are ol and ol + pl phyric. Microprobe analyses of the tholeiitic suite phenocrysts indicate that they are Fo68-Fo86, An58-An75, and augite (Ca34Mg50Fe16). The range of olivine and plagioclase compositions represents the chemical variation of the phenocryst compositions with fractionation. The phenocyrsts in the alkali basalts are Fo81 and An69. The suite of tholeiites comprises a fractionation series characterized by relative enrichment of Fe, Ti, Mn, V, Na, K, and P and depletion of Ca, Al, Mg, Ni, and Cr. The fractionated tholeiites occur on the median ridge (which is a sliver of normal oceanic crust) of the double Siqueiros transform fault, on the western Siqueiros fracture zone, and on the adjoining East Pacific Rise, while the two transform fault troughs contain mostly unfractionated or only slightly fractionated tholeiite. We suggest that the fractionated tholeiites are produced by fractional crystallization of more 'primitive' tholeiitic liquid in a crustal magma chamber below the crest of the East Pacific Rise. This magma chamber may be disrupted by the transform fault troughs, thus explaining the paucity of fractionated tholeiites in the troughs. The alkali basalts are found only on the flanks of a topographic high near the intersection of the northern transform trough with the East Pacific Rise.从锡凯罗斯转换断层/断裂带（Siqueiros transform fault/fracture zone）及邻区东太平洋海隆（East Pacific Rise）采集的玄武岩样品，以Fe³+/Fe²+≈0.25的新鲜大洋拉斑玄武岩（oceanic tholeiite）及分异型大洋拉斑玄武岩为主，但区域内亦产出碱性玄武岩（alkali basalts）。拉斑玄武岩系列的岩石为橄榄石-斜长石斑晶型及橄榄石-斜长石-单斜辉石斑晶型玄武岩，而碱性玄武岩则为纯橄榄石斑晶型与橄榄石-斜长石斑晶型。对该拉斑系列斑晶（phenocrysts）的电子探针分析（microprobe analyses）显示，其橄榄石组分范围为Fo68-Fo86，斜长石组分范围为An58-An75，单斜辉石则为Ca₃₄Mg₅₀Fe₁₆型普通辉石（augite）。橄榄石与斜长石的成分区间反映了斑晶成分随分异作用产生的化学变化。碱性玄武岩中的斑晶则为Fo81与An69。拉斑玄武岩系列构成了一套以Fe、Ti、Mn、V、Na、K、P相对富集，Ca、Al、Mg、Ni、Cr相对亏损为特征的分异序列。分异型拉斑玄武岩分布于锡凯罗斯转换断层的中央脊（为正常大洋地壳的碎片）、西部锡凯罗斯断裂带及毗邻的东太平洋海隆，而两处转换断层槽内则以未分异或仅弱分异的拉斑玄武岩为主。我们认为，分异型拉斑玄武岩是由东太平洋海隆轴部下方地壳岩浆房（crustal magma chamber）中更“原始”的拉斑质岩浆经分离结晶作用（fractional crystallization）形成的。该岩浆房可能因转换断层槽的活动而遭到破坏，这也解释了转换断层槽内分异型拉斑玄武岩较为稀少的现象。碱性玄武岩仅分布于北部转换槽与东太平洋海隆交汇点附近的一处地形高地侧翼。