Geochemistry of mafic clasts of ODP Holes 125-778A and 125-779A

Clasts of metamorphosed mafic igneous rock of diverse composition were recovered in two drill sites on a serpentine mud volcano in the outer Mariana forearc during Ocean Drilling Program Leg 125. These clasts are xenolithic fragments that have been entrained in the rising serpentine mud, and make up less that 9% of the total rock recovered at Sites 778 and 779. Most samples are metabasalt or metadiabase, although one clast of possible boninite and one cumulate gabbro were recovered.
On the basis of trace element signatures, samples are interpreted to represent both arc-derived and mid-ocean ridge-derived compositions. Rocks with extremely low TiO2 (<0.3 wt%) and Zr (<30 ppm) are similar to boninite series rocks. Samples with low TiO2 (<0.9 wt%) and Zr (<50 ppm) and extreme potassium enrichment (K2O/Na2O >3.9) may represent island arc rocks similar to shoshonites. However, the K2O/Na2O ratios are much higher than those reported for shoshonites from modem or ancient arcs and may be the result of metamorphism. Samples with moderate TiO2 (1.4 to 1.5 wt%) and Zr (72 to 85 ppm) are similar to rocks from mid-ocean ridges. A few samples have TiO2 and Zr intermediate between island arc and mid-ocean ridge basalt-like rocks. Two samples have high iron (Fe2O3* = >12.8 to 18.5 wt%) (Fe2O3* = total iron calculated as Fe2O3) and TiO2 (>2.3 wt%) and resemble FeTi basalt recovered from mid-ocean ridges. Metamorphism in most samples ranges from low-temperature zeolite, typical of ocean floor weathering, to prehnite-pumpellyite facies and perhaps lower greenschist. Blue amphibole and lawsonite minerals are present in several samples. One diabase clast (Sample 9) exhibits Ca enrichment, similar to rodingite metamorphism, typical of mafic blocks in serpentinized masses. The presence of both low-grade (clays and zeolites) and higher grade (lawsonite) metamorphism indicates retrograde processes in these clasts.
These clasts are fragments of the forearc crust and possibly of the subducting plate that have been entrained in the rising serpentine and may represent the deepest mafic rocks ever recovered from the Mariana forearc. The variable compositions and degree of metamorphism of these clasts requires at least two tectonic origins. The recovery of clasts with mid-ocean ridge and arc chemical affinities in a single drill hole requires these clasts to have been "mixed" on a small scale either (1) in the forearc crustal sequence, or (2) after inclusion in the rising serpentine mud. The source of the MORB-like samples and an explanation for the presence of both MORB-like and arc-like rocks in close proximity is critical to any model of the evolution of the Mariana forearc. The source of the MORB-like samples likely will be one (or more) of the following: (1) accretion of Pacific plate lithosphere, (2) remnants of original forearc crust (trapped plate), (3) volcanism in the supra-subduction zone (arc or forearc) environment, or (4) derivation from the subducting slab by faulting along the dÈcollement.

在大洋钻探计划（Ocean Drilling Program）第125航次期间，科研人员于马里亚纳弧前外缘的一座蛇纹石泥火山的两个钻探站位，采获了成分多样的变质镁铁质火成岩岩屑。这些岩屑为被上升的蛇纹石泥裹挟的捕虏岩碎屑（xenolithic fragments），在778和779站位的总采获岩石中占比不足9%。绝大多数样品为变玄武岩（metabasalt）或变辉绿岩（metadiabase），仅采获1块疑似玻安岩（boninite）岩屑与1块堆积型辉长岩（cumulate gabbro）岩屑。
基于微量元素特征（trace element signatures），这些样品被认为兼具弧源型与洋中脊源型（mid-ocean ridge-derived）成分。其中，二氧化钛（TiO₂）含量极低（<0.3 wt%）且锆（Zr）含量<30 ppm的岩石，与玻安岩系列岩石特征相似；TiO₂含量<0.9 wt%、Zr<50 ppm且钾极度富集（K₂O/Na₂O>3.9）的样品，可能属于与钾玄岩（shoshonites）类似的岛弧岩石。不过，其K₂O/Na₂O比值远高于现代或古岛弧钾玄岩的报道值，这一现象可能由变质作用导致。TiO₂含量中等（1.4~1.5 wt%）、Zr含量72~85 ppm的样品，与洋中脊岩石特征相近。另有少量样品的TiO₂与Zr含量介于岛弧与洋中脊玄武岩（mid-ocean ridge basalt, MORB）类岩石之间。2个样品的全铁（以Fe₂O₃计，Fe₂O₃*）含量>12.8%~18.5 wt%且TiO₂含量>2.3 wt%，与从洋中脊采获的FeTi玄武岩相似。多数样品的变质程度涵盖海底风化（ocean floor weathering）典型的低温沸石相（low-temperature zeolite），至葡萄石-绿纤石相（prehnite-pumpellyite facies），甚至可能低绿片岩相（lower greenschist）。部分样品中可见蓝闪石（blue amphibole）与硬柱石（lawsonite）矿物。1件辉绿岩岩屑（diabase clast，样品9）表现出钙富集特征，与蛇纹石化岩体（serpentinized masses）中镁铁质岩块典型的杆石变质作用（rodingite metamorphism）相似。同时存在低级别变质（黏土与沸石）与高级别变质（硬柱石）的现象，表明这些岩屑经历了退变质作用（retrograde processes）。
这些岩屑为弧前地壳（forearc crust）乃至俯冲板块（subducting plate）的碎片，被上升的蛇纹石泥裹挟，可能是迄今从马里亚纳弧前采获的最深部镁铁质岩石。其成分与变质程度的多样性，表明至少存在两种构造成因。在单个钻孔中采获兼具洋中脊玄武岩与岛弧地球化学亲和性的岩屑，说明这些岩屑曾在小尺度范围内发生混合：要么（1）在弧前地壳层序中，要么（2）在被裹挟进入上升的蛇纹石泥之后。洋中脊玄武岩类岩屑的成因，以及为何洋中脊源与岛弧源岩屑能够近距离共存，是马里亚纳弧前演化任何模型的关键。洋中脊玄武岩类岩屑的成因可能为以下一种或多种：（1）太平洋板块岩石圈（Pacific plate lithosphere）的增生作用，（2）原始弧前地壳（捕获板块，trapped plate）的残余体，（3）俯冲带上方（supra-subduction zone，弧或弧前）环境中的火山作用，或（4）沿拆离面（dÉcollement）断裂从俯冲板片剥离的物质。