Partial Melting and Assimilation of the Roof of Melt Lens, and New Perspectives of Hydrothermal Systems Beneath Fast-Spread Ocean Ridges

Abstract:

Numerous anatectic blocks derived from a sheeted dike complex appear in a block-rich zone between the foliated gabbro and a large plagiogranite complex in the Oman ophiolite. Five blocks from blocks 1 to 5 in a descending stratigraphic order are studied. Host rocks are quart diorite (block 1), diorite (block 2), massive gabbronorite (block 3), and foliated gabbronorite and olivine gabbronorite (blocks 4 and 5). The textural and petrographical features vary depending on the depths of the blocks. Block 1 preserves a doleritic texture despite of the presence of a granulite facies mineral assemblage. Abundant mafic and quartz globules in blocks 2 and 3 suggest processes of in-situ partial melting and melt accumulation. Hornblende occurs in the globules but is absent from the matrix. Hornblendes and quartz decrease with increasing depth, suggesting the progressive extraction of partial melts from the blocks. Assimilation of the host gabbro by the expelled silicic melts is shown by the co-existence of quartz and olivine. The SiO₂ and trace element contents of the blocks decrease with increasing depths. With decreasing REE contents, there is an increase in positive Eu anomalies, and the REE patterns vary from flat to patterns that are depleted in light REEs. The rates of partial meting evaluated by a mass balance equation and trace element contents increase with depths.

Inverted pigeonite appears in the blocks 3 to 5, and is absent at a shallower level of the blocks 1 and 2. The thermal gradient in the block-rich zone, ~100 m thick, was very high more than ~1°C/1m. At the shallower level of the block-rich zone, the temperatures do not attain the minimum stability limit of pigeonites, ~ 965 °C. Elevating temperatures with depth result in the appearance of pigeonites. However, they are limited inside the blocks due to varying Mg# values within the blocks. Efficient extraction of partial melts, and/or intensive effects of assimilation with gabbroic magmas at the margin of the blocks caused such Mg# variations. Therefore, pigeonites disappear in the margin, whereas they persisted inside the blocks. Thus, appearance of pigeonites due to increasing temperatures with depths and disappearance of pigeonites due to increasing Mg# within the blocks are encountered.