Petrology and geochemistry of the basement boninite series of ODP Site 125-786

The 720 m of igneous basement that was penetrated at Site 786 of Ocean Drilling Program Leg 125 consists of boninite-series volcanics. Bronzite andesites dominate the lithology and primitive magmas of high-Ca, intermediate-Ca, and low-Ca boninite are present in subordinate amounts. Sparsely phyric boninites typically contain olivine and orthopyroxene phenocrysts with Mg numbers [= Mg/(Mg + Fe) in moles] between 86% and 87%. Their high whole-rock Mg numbers, and the absence of zonation in the phenocrysts, imply equilibration at temperatures probably between 1200° and 1250°C, and 20° to 50°C below their liquidus. Equilibrium olivine and orthopyroxene have identical Mg numbers, and Mg/Fe partitioning between these minerals and the melt thus can be described with a single Kd. The invariably phenocryst-rich bronzite andesites contain Plagioclase that has spectacular zoning and mafic phases that can be as magnesian as those of the boninite parent. The most evolved melts are rhyolites with hypersthene, Plagioclase (An50), and magnetite. Eruption temperatures for the rhyolites are estimated at about 1000°C. Some magmas contain ferroactinolite in the groundmass, which is most likely a secondary, low-temperature phase.
The locally large contrasts in degree of alteration are consistent with multiple episodes of magmatic activity. However, all igneous events produced boninite volcanics. Only the first, the edifice-building episode, gave rise to differentiated magmas. Differentiation of parental boninites took place by limited fractional crystallization, producing bronzite andesites. The erupted andesites, dacites and rhyolites are filter pressed extracts from these bronzite andesite magmas, which, as a result, have accumulated crystals. Subsequent younger igneous events produced high-Ca and intermediate-Ca boninites which intruded as dikes and sills throughout the basement sequence. The mineralogy of the dikes and sills reflects variable degrees of subliquidus cooling of the magma before emplacement.

大洋钻探计划（Ocean Drilling Program, ODP）第125航次786站位钻遇的720米火成基底由玻安岩系火山岩（boninite-series volcanics）构成。岩性以古铜安山岩（bronzite andesites）为主，高钙、中钙及低钙玻安岩对应的原始岩浆仅占次要占比。斑晶稀疏的玻安岩通常含有橄榄石（olivine）与斜方辉石（orthopyroxene）斑晶，其镁值（Mg numbers，摩尔比Mg/(Mg+Fe)）介于86%~87%之间。全岩高镁值以及斑晶无环带的特征，指示其形成温度大致介于1200℃~1250℃，较其液相线温度低20℃~50℃，达到化学平衡状态。平衡状态下的橄榄石与斜方辉石具有完全一致的镁值，因此二者与熔体之间的镁铁分配关系可通过单一分配系数（Kd）进行描述。普遍富含斑晶的古铜安山岩含有具显著环带结构的斜长石（Plagioclase），其伴生的铁镁矿物相的镁含量可与玻安岩母岩浆相媲美。演化程度最高的熔体为含紫苏辉石（hypersthene）、钙长石牌号An50的斜长石以及磁铁矿（magnetite）的流纹岩，其喷发温度估算约为1000℃。部分岩浆的基质中含有铁阳起石（ferroactinolite），该矿物大概率为次生低温成因相。
局部区域的蚀变程度存在显著差异，这与多期岩浆活动的特征高度吻合。不过所有岩浆事件均形成了玻安岩系火山岩，仅第一期（建造火山锥期）岩浆事件产生了分异岩浆。母质玻安岩通过有限的分离结晶（fractional crystallization）作用发生分异，形成古铜安山岩。已喷发的安山岩、英安岩与流纹岩均为源自该类古铜安山岩岩浆的滤压萃取物，因此岩浆内部发生了晶体堆积。后续的较年轻岩浆事件形成了高钙与中钙玻安岩，这些玻安岩以岩墙（dikes）和岩床（sills）的形式侵入整个基底序列中。岩墙与岩床的矿物学特征反映了岩浆在侵位前经历了不同程度的亚液相线冷却（subliquidus cooling）。