A field, petrographical and geochemical investigation of the Kennack Gneiss, Lizard Peninsula, southwest England

Sandeman, Hamish A. I. (1988) A field, petrographical and geochemical investigation of the Kennack Gneiss, Lizard Peninsula, southwest England. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Lizard complex of southwest England is considered to represent a thrust slice of Devonian oceanic crust, emplaced onto the Gramscatho flysch basin during the Middle Devonian. The ophiolite is an incomplete sequence consisting of strongly deformed basaltic lavas and interflow sediments (Landewednack and Old Lizard Head Series schists respectively), overlain by serpentinized peridotites and lherzolites, variable gabbro and a restricted sheeted dyke complex. Intruded into the basal, serpentinized ultramafic rocks are a suite of compositionally banded rocks called the Kennack Gneiss. Excluding some late mafic dykes, the Kennack Gneiss is the youngest rock suite present, because portions of the gneiss intrude and cross-cut the foliations of all the other rock types. -- The banded Kennack Gneiss consists of distinct felsic and mafic fractions. Field relationships with other units indicate an intrusive igneous nature for the Kennack Gneiss and the relationships between the felsic and mafic fractions of the gneiss suggest that magma-mingling and possibly mixing have played a significant role in generation of these rocks. Digestion of plastic gabbroic xenoliths by felsic material, net-veining of silicic material in microgabbro and flame-like interfingering of felsic and mafic fractions all support the proposal of magmatic mingling. -- Field relationships also indicate that the mafic fraction of the Kennack Gneiss is distinct from the hornblende schists, and that some mafic dykes are genetically related to the mafic fraction of the Kennack Gneiss. Variably deformed pegmatitic gabbro within the map area appears to be only slightly older than the Kennack Gneiss and is significantly different in chemistry from the Crousa gabbro. -- Harker variation diagrams for the Kennack Gneiss as a whole show variable trends. The presence of curvilinear trends indicates that a restite separation or in situ metamorphic segregation processes may be discounted as viable hypotheses. Instead, fractional crystallization, thermogravitational diffusion or magma-mixing are favored. The trace element chemistry of the Kennack gneiss indicates that magma-mixing is the most favorable of the three. This is demonstrated through irregular inter end member trends not easily explained through traditional fractional crystallization processes. -- Negative colinear variations with silica dominate the felsic fractions of the Kennack Gneiss. MgO variation diagrams for the mafic fractions of the gneiss show a high degree of scatter, but particular elements (CaO, Na₂O, SiO₂ and Sr) exhibit curvilinear covariance with MgO. These points suggest that the felsic fractions are interrelated through mixing, while the mafic fractions are interrelated through progressive fractional crystallization. This indicates that the chemical and physical evolution of the felsic fractions has been dominated by magmatic mingling and mixing of a silicic magma with a late stage differentiate of the mafic fractions. Although the majority of the chemical data for the felsic rocks supports mixing, a number of elements, Y, Nb, possibly Th and U and the rare earth elements exhibit strong enrichment/depletion trends which are difficult to interpret. However, careful interpretatiqn indicates that the felsic fractions of the gneiss were initially generated through partial melting of a crustal source, intermediate in composition which contained abundant plagioclase and a HFSE bearing phase such as zircon. These crustal melts underwent crystal fractionation of alkali feldspar as well as a LREE bearing phase (monazite?), as indicated by the extreme enrichment of the Y, Nb and possibly the HREE in the most evolved silicic rocks. Following this crystal fractionation, the felsic magma was intruded into a magma chamber containing the mafic fraction of the gneiss. Mixing of the felsic magma and the most differentiated portions of the mafic magma occurred, however, mixing was only local as demonstrated by the mingled state now observed in the field. The mingled and partially mixed magmas were then intruded along the base of and into the Lizard peridotite. Intrusion of the Kennack Gneiss was synchronous with displacement of the Lizard complex from the oceanic regime. The strongly banded character of the Kennack Gneiss resulted from a high strain gradient at the base of the Lizard complex at that time. -- Major element chemistry of the mafic fractions suggest a volcanic arc influence in generation of these rocks. Similarly, trace and rare earth element chemistry in combination with tectonic discrimination diagrams support a volcanic arc setting of generation for the two magmas. These observations have significant implications for the geotectonic history of the Lizard complex.

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