PETROGENESIS OF THE CENTRAL GABBROIC PLUTON IN THE LAKE ZONE, WESTERN MONGOLIA

Abstract

The Central pluton in the Lake Zone, western Mongolia, is a shallow-seated layered basic intrusion of the island-arc type. The pluton belongs to the Cambrian peridotite-pyroxenite-anorthosite- gabbronorite rock association, which is widespread in western Mongolia. It is composed of chilled facies, marginal facies, and the layered series including ultramafic, subultramafic, mafic, and anorthositic rock groups. As follows from the character of layering, the pluton was formed in a single stage through the emplacement of an initial magma batch. The distribution of major and minor elements in cumulates and residual melt was modeled using the COMAGMAT 3.5 program. The results of calculations agree best with the real rock compositions at the following parameters: a pressure of 1 kbar, up to 0.5 wt % H2O in the melt, and a QFM buffer. The model primary melt contains 20 wt % MgO and 7 wt % FeO; 100Mg/(Mg + Fe) = 83.59. It was established that the initial magma that filled the intrusive chamber already entrained about 20% of olivine crystals, which settled to the bottom of the chamber making up a basal peridodite unit. Olivine crystallized at a shallow depth in a transitional magma chamber. The rocks of the Central pluton and their parental melt are characterized by low REE contents similar to those of the primitive mantle and the N-MORB source with Ta, Hf, Ti minima and U and Th maxima; a marked depletion in HREE is typical. The relatively high U and Th contents, slight enrichment in LREE with (La/Yb) ch = 1.60-1.91, and Ta minimum in all rocks resemble the geochemical signature of the island-arc tholeiitic series. The distribution of HFSE and HREE, which are immobile during the dehydration of a subducted oceanic plate, characterizes the mantle matter similar to the N-MORB source, which existed before enrichment in mobile elements during subduction. The primary melt composition was modeled by the MELTS program. It corresponds to the melt that was formed through the batch partial melting of the depleted mantle source (garnet lherzolite) at a pressure of 25 kbar, a temperature of 1600°C, 0.1 wt % H 2O in the source, and a degree of melting of ∼20%. The Central pluton was emplaced in a primitive island-arc environment. However, it cannot be regarded as a direct hypabyssal comagmatic counterpart of the Cambrian high-alumina volcanics of the Lake Zone, because the latter have been contaminated by crustal materials and underwent magma mixing. The general geochemical similarity of volcanic and plutonic rocks supports the suggestion that a common subduction-related garnet-bearing mantle source exists.