Primitive layered gabbros from fast-spreading lower oceanic crust

Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks-in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas-provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt

飛騨外縁帯，小滝地域の超苦鉄質岩: 大江山オフィオライトかんらん岩とその変成作用について

金沢大学理工研究域自然システム学系飛騨外縁帯の小滝地域に分布する超苦鉄質岩は2タイプに大別され，それらは（Type 1） 初生的なかんらん岩，（Type 2）変成かんらん岩である．これらの一部は接触変成作用を被っている．同地域の超苦鉄質岩中の初生的な残存鉱物は，大江山オフィオライトのものに類似し，大江山岩体以外では初のAlに富むぜん虫状スピネル（Cr#＝33～38）を含むものもある．変成かんらん岩は，Naに富むトレモラ閃石を含むことで特徴づけられるが，このことはこのタイプの超苦鉄質岩が沈み込むスラブ由来の流体による加水作用に関連した元素の移動によって交代作用を被った可能性を示唆する．接触変成作用を被ったと考えられる超苦鉄質岩は，一般に本地域の東部に向かってより高温で安定な鉱物組み合わせを示し，岩体東部を不整合に覆う石坂流紋岩直下の深成岩体による接触変成作用を示唆する．Ultramafic bodies in the Kotaki area of the Hida Marginal Belt are composed of Paleozoic ophiolites and Paleozoic-Mesozoic sediments. The ultramafic rocks can be classified into two major types. Type 1 consists of primary ultramafic rocks that preserve primary textures and primary minerals of mantle peridotite. Type 2 consists of regionally metamorphosed peridotite that has a schistose or mylonitic texture. Both types were locally affected by later contact metamorphism.The primary mantle peridotite (Type 1) is subdivided into a high-Al group [spinel Cr#, Cr/ (Cr+Al), of 0.33-0.38] and a high-Cr group [Cr# of 0.48-0.55]. The high-Cr group is pervasive among ultramafic bodies of the Oeyama ophiolite. The high-Al group is identical to lherzolitic peridotite that has been found only in the Oeyama ultramafic body.Regionally metamorphosed peridotite (Type 2) can be subdivided into two subtypes based on mineral assemblage and texture. Type 2A contains olivine (ol) +tremolite (tr) ±antigorite (atg) ±orthopyroxene (opx). Type 2B contains ol+atg+clinopyroxene (cpx). Types 2A and 2B are similar to the peridotite mylonite and serpentinite mylonite, respectively, reported from the Happo ultramafic body to the south. Type 2A metaperidotite contains Na-rich tremolite (up to 2.53 wt.% Na2O) produced by metasomatism, possibly related to the slab-derived fluid that penetrated through the wedge mantle above an early Paleozoic subduction zone. Type 2B metaperidotite may represent the wedge mantle metamorphosed at a lower temperature, and Type 1 peridotite may represent the mantle portion that was unaffected by metamorphism and metasomatism.Serpentinite hornfelses that formed by later contact metamorphism vary in their mineral assemblages, defined by atg, ol+cpx, ol+tr, ol+tr+talc, and ol+tr+opx zones that are arranged from west to east. The zones indicate that metamorphic temperatures generally increased toward the east. This finding suggests that the hornfelsic metamorphism of previously serpentinized peridotites of Type 1 and Type 2 was caused by a concealed plutonic intrusion beneath the Ishizaka rhyolite, which is exposed to the east

北海道渡島帯，ジュラ紀付加体中の小砂子緑色岩体の褶曲構造

金沢大学理工研究域自然システム学