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Magma Genesis in Kabanjahe Region Continental Margin Arc of Sumatra
Authors: B.H. Harahap
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Volcanic rocks in Kabanjahe region, Karo Regency, North Sumatra Province, are products of old Toba Caldera, Sibayak Volcano, and Sipiso-piso Volcano. Rhyolitic tuff is the main lithology distributed over a large area in this region. Others are basaltic, basaltic andesitic, andesitic, dacitic, and rhyolitic lavas. Data show that
the rock was originated from magma of a continental origin formed at a subduction zone environment. Petrogenetic
modelling suggests that the range in composition was mainly controlled by a fractional crystallization of plagioclase, clinopyroxene, hornblende, and biotite. Harker’s variation diagram of major and trace elements
show a continuous range that indicates they are cognate. The lava in this area belongs to a high-K, calc-alkaline
series, with particular high Nb concentrations. The composition of these high-Nb lavas is more similar to those
of intra plate basalts rather than those of calc-alkaline or arc-tholeiitic basalt. The high anomaly of Nb which
is accompanied by high Th, Rb, and normative corundum suggests that the source may also be enriched in
incompatible elements, a characteristic feature of alkali magmatism. The similarity of the trace element of
volcanic rocks to the within-plate basalts indicates that the convecting mantle wedge above subducted slabs contains variable proportions of MORB-source and OIB-source components; fluids added were derived from the subducted slab. Hence, it is interpreted that the high Nb concentration of volcanic rocks from Kabanjahe region were generated from subduction modified OIB source components. Alternatively, a deep seated faulting conduit magma from the lower mantle resulted in the alkaline enrichment of the volcanics. This article performs a petrological aspect, especially based on geochemical analysis including major elements, trace elements, and rare earth elements. The results are plotted into a general and specific classification used in petrology.
Keywords: petrology, geochemistry, Kabanjahe, continental margin arc, tectonic