Marginal reversal in marginal zone of mafic-ultramafic layered intrusions, insights from the Mazhalyk peridotite-gabbro intrusion (South-Eastern Tuva)

Marginal zones of igneous bodies are the first products of magma crystallization produced during the initial stage of magma emplacement into magma chambers. This stage is very important for petrological studies of intrusive rocks, but still remains, however, insufficiently understood. The progress in its understanding is significantly hampered by the paucity of critical observations on the marginal zones of igneous bodies. To fill this gap, we have undertaken a detailed petrographic, geochemical and especially mineralogical study of marginal zones of the Mazhalyk peridotite-gabbro layered intrusion (South-Eastern Tuva, Russia). The study has resulted in observations that provide new constraints on and insights into the processes operating at magma chamber margins during the initial filling, subsequent crystallization and solidification of basaltic magma chambers. The rocks of marginal zone of Mazhalyk intrusion become more primitive from the base upwards as exemplified by a significant increase in whole-rock MgO (from 2,9 to 10 wt. %), Mg-number (from 41 to 57 %), and normative An-content (from 80 to 95 %) whereas all incompatible components reveal an upward decrease, e.g. TiO₂ (from 0,32 to 0,15 wt. %), Na₂O (from 1,82 to 0,1 wt. %), P₂O₅ (from 0,04 to 0,01 wt. %). The reverse trends are also evident from an upward increase in the An-content of plagioclase (from 81 to 91 %) and Mg-number of clinopyroxene (from 70 to 83 %) and amphibole (from 62 to 79 %). The marginal zone ends abruptly at the base of the Layered Series. The boundary between these two major units is a sharp break in terms chemical composition, mineral composition and crystallization sequence. The marginal zone can thus be viewed as an example of an aborted reversal. The Layered Series of the Mazhalyk intrusion show inward decreases in whole-rock MgO, Mg-number, and normative An-content as expected from normal fractional crystallization. A major result of this study is that the formation of the marginal reversal of the Mazhalyk intrusion was from inflowing magmas which become more primitive in composition with time. The data indicate that filling of the chamber started with evolved liquids that likely represent the leading fractionates of parental basaltic magma that crystallized against sidewalls of a deep conduit system or in deep magma chamber. Then inflowing magmas become more primitive (have higher MgO) and marginal zone crystallizing from such inflowing magmas acquires features of a typical marginal reversal, with rocks and minerals becoming more primitive in composition inwards. This is followed by a new major influx of a primitive magma that terminates the development of a marginal reversal and restarts crystallization with more primitive mineral and rock compositions of the Layered Series. This results in the formation of a sharp compositional break of the Layered Series with the underlying rocks of marginal reversal.

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