The origin of rhythmic magmatic layering in coarse-grained porphyritic S-type granite of the Peninsula pluton, Cape Granite Suite, South Africa

[摘要] ENGLISH ABSTRACT: Rhythmic magmatic layering in granites is an intriguing feature that has been reported from plutons with contrasting chemical compositions from a wide range of tectonic settings. Layered granites are rare and occur in association with volumetrically dominant non-layered rocks having similar composition. Understanding the origin of such layering in granites, particularly from well exposed outcrops, may give crucial insights on the physical-chemical conditions and processes (such as fractional crystallization, size and composition of individual magma batches, efficiency of mixing between batches etc.) within magma chambers during their construction. In the Cape Granite Suite, rhythmic layering is exceptionally well preserved in outcrops of the S-type granodioritic to monzogranitic Peninsula pluton along the coastline of the small town of Llandudno.At Llandudno, the granodioritic facies of the Peninsula pluton is exposed as a coarse-grained, cordierite and K-feldspar phenocrystic granite (referred to as Llandudno granodiorite in this study). Layering within the Llandudno granodiorite occurs within several lens-shaped bodies, of which the largest, with a thickness of about 5 m. The studied rhythmic sequence comprises 50 layers with thicknesses ranging between 5 and 50 cm. Each layer is typically characterised by a sharp lower contact overlain by a biotite-rich portion containing approximately 50 vol. % biotite. This mafic lower portion, grades upwards into a strongly leucocratic upper portion enriched in quartz, plagioclase and K-feldspar. The layered rocks host a substantially lower abundance of K-feldspar megacrysts and cordierite than the surrounding Llandudno granodiorite. In particular, cordierite crystals are five times less abundant in the layered zone than the un-layered Llandudno granodiorite. The K-Feldspars megacrysts that do occur within the layering are generally concentrated in the mafic part of some of the layers and are commonly oriented parallel to the layering. The K-feldspar megacrysts are significantly smaller than those in Llandudno granodiorite, but were found to be similar in composition.The whole rock compositions of the layered rocks are peraluminous with A/CNK >1.4. They have lower Mg#s (51 to 58) than the Llandudno granodiorite (51 to 65). The Na2O/CaO ratio of mafic and leucocratic portions is higher than in the Peninsula pluton. The mafic portions show an enrichment of trace and rare earth elements relative to the leucocratic portions and Peninsula pluton. The difference in bulk rock composition as well as trace and rare earth elements composition shows that the layered rocks were not formed by magmas produced by differentiation of the Llandudno granodiorite. The differences in biotite composition in basal sections of adjacent layers suggest that each layer represents a separate magma pulse, with the mafic portions of the layers largely representing an accumulation of the crystals in the magma batch at the time of injection and that these crystals mostly consist of orthopyroxene and biotite. This is in agreement with the findings based on the whole rock chemistry of the layers compared with the Llandudno granodiorite. The inclusion of K-feldspar megacrysts from the host granite into the layering, as well as the exceptional state of preservation of the layering, suggests that the layering formed relatively late in the crystallization sequence of the granite. This, in combination with the evidence for subtle differences in the chemistry of the magmas that formed separate layers, suggests that the layers represent a frozen feeder zone in the Peninsula pluton recording small successive pulses of magma addition. Frequent occurrence of the less-preserved layered biotite schlieren in the pluton may represent the equivalents of the layering that have become partially digested and texturally equilibrated with the host magma.

[发布机构] Stellenbosch University