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http://dx.doi.org/10.17014/ijog.vol3no1.20083Volcanic rocks of Mount Tampomas and its surroundings belong to low–K, calc–alkaline, and high–K series of basalt, basaltic andesite, and andesite suites containing 48.59 – 60.49 wt% of SiO and 0.31-1.67 wt% of K O. The rocks are typically formed in an island arc environment characterized by the high content of Al O (16.47 – 21.76 wt%), low to moderate content of MgO (2.37 – 8.84 wt%), low content of both titanium TiO (< 1 wt%) and high field strength elements (HFSEs), negative anomaly of Nb relative to K and La. The light rare earth elements (LREEs) enrichment but low abundances of heavy rare earth elements (HREEs) indicate fractionation process. The parent magma was an extensive removal of olivine and clino-pyroxene, plagioclase, apatite, and titanoferrousmagnetite from a basalt olivine magma of N-MORB character that was firstly enriched by LILEs such as Ba, Rb, Th, K, and LREE (La). The differentiation lasted, and the magma became water undersaturated at the crystallization phase of hornblende. The high content of incompatible elements such as Ba, Th, La, Ce, may indicate contamination by liquid derived from the subducted slab.

High-MgO (>12 wt%) magmas represent some of the most primary and high temperature melts from mantle plumes. The compositional diversity of high-MgO magmas gained by fractional melting within inhomogeneous mantle sources, is often overprinted by wall rock assimilation, magma mixing and fractional crystallisation within magma chambers at various depths within the crust. The deepest intrusions, at the base of the crust, are the first stop for magmas as they leave the mantle and such localities offer vital insights into the crustal processes that modify high-MgO melts. The Seiland Igneous Province (SIP) represents a rare exposure of a deep crustal magma conduit system that transported large volumes of mantle-derived melts through the lower continental crust. In this work, trace element compositions of clinopyroxenes in dunite, wehrlite and olivine clinopyroxenite samples from the Reinfjord intrusion were measured using laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The melts calculated to be in equilibrium with the clinopyroxenes represent derivatives of mantle-derived magmas and have steep rare earth element (REE) profiles with La/Yb PM of 8.4–14., Sm/Yb PM of 3.5–4.8, and negative anomalies in Nd, Zr and Hf. Assimilation, recharge and fractional crystallisation modelling shows that the ultramafic cumulates in the Reinfjord intrusion formed through concurrent fractional crystallisation and repetitive recharge (and mixing) of new primitive magmas from the mantle with very limited input from crustal sources. The recharge and mixing model is strongly supported by field and petrographic data. Two end-member ultramafic magmas are identified; the first end-member melt composition (MELT CPX0) is similar to the melt compositions that are in equilibrium with the most trace element depleted clinopyroxenes from Reinfjord. The second end-member melt composition (MELT PIC0) is similar to a number of picrite dykes found throughout the SIP, which have less steep LREE/HREE and lack strong negative anomalies in Zr and Hf. The REEBOX PRO melting model (Brown and Lesher, 2016) was used to forward model adiabatic decompression melting of lithologically homogeneous and heterogeneous sources containing anhydrous/hydrous peridotite ± pyroxenite. It was found that MELT PIC0 formed by melting of a peridotite source with up to 10% pyroxenite component and mantle potential temperatures of up to 1450 °C. MELT CPX0, however, formed by melting of a carbonate- metasomatised peridotite with potential temperatures of up to 1630 °C. These results indicate a compositionally and thermally inhomogeneous mantle plume beneath the SIP. • Trace element compositions of clinopyroxene in ultramafic cumulates from Reinfjord. • Parent melts were high-MgO picrites with OIB affinities. • Recharge, magma mixing, and fractional crystallisation during cumulate formation. • Inhomogeneous mantle plume source with DMM, carbonated peridotite and pyroxenite. [ABSTRACT FROM AUTHOR]