Macalester College Geology Department

Character and Significance of a Silicified Unconformity in Late Triassic-Early Jurassic Strata of the Limpopo Valley, Southern Zimbabwe

The Limpopo Valley of southern Zimbabwe preserves well-exposed strata spanning the Permian to Early Jurassic, including the Fulton's Drift, Gushu, Mpandi and Samkoto Formations, capping the sedimentary succession are the Tuli Basalts. Despite years of intensive study of the South African Karoo system, the Limpopo Valley has remained largely unexplored from both a paleontological and geological perspective.

Our recent fieldwork in this area resulted in the discovery of a previously unrecognized, laterally continuous erosional unconformity embedded between the Mpandi Formation and superjacent Samkoto Formation. This unconformity, which is characterized by scour topography, rests upon the 2-m-thick silicified cap of the Mpandi Formation. XRF analyses reveal an increased concentration of SiO2 in the silicified layer, and depletion of major and trace elements in the silicified layer compared to the background Mpandi sediments. XRD analyses of clay minerals indicate an absence of a deep kaolinite or illite weathering profile directly below the silcrete. The lateral continuity and characteristic profile of the layer (globular base grading up into prismatic and pseudobreccia horizons) suggest formation at the surface as a result of pedogenic processes. Within a predominantly GS and M-Fabric matrix, length fast and length slow chalcedony are present, with vug fills dominated by microcrystalline quartz, length fast chalcedony, and macrocrystalline quartz. Illuviation structures such as colloform features typically formed by the movement of groundwater are noticeably absent.

Geochemical data, petrographic evidence, and field relationships indicate pedogenic formative processes active in an arid to semi-arid climate regime. This interpretation is consistent with studies of the South African Karoo system that indicate a Late Triassic-Early Jurassic regional shift to an increasingly arid climate regime under stable tectonic conditions. The unconformtiy, along with a new radioisotopic age date from the Tuli Basalts, provide much neeeded age control for the Limpopo Valley succession, especially as it pertains to correlation with South African Karoo strata.

A Fusion Based Method of Whole Rock Dissolution for ICP-Mass Spectrometry and the Origin of Midcontinent Rift Granophyres

Inductively coupled plasma-mass spectrometry (ICP-MS) is a preferred method of rare earth element analysis but requires complete sample dissolution to obtain accurate results. Low pressure, low temperature acid digestion is a common method of dissolution employed in sample preparation, but refractory minerals (e.g. zircon) may not completely dissolve. Experiments performed at Macalester College have shown fusion with a lithium metaborate flux and subsequent dissolution in dilute nitric acid to be a reliable and efficient method of sample preparation for chemically resistant rocks such as granites. For the granitic rocks of the Midcontinent rift, experimentation showed a rock:flux ration of 1:4 (0.4 g rock powder) to consistenly result in complete dissolution of the sample while minimizing sample dilution and sampling error. Trace element analyses of several international standards (AC-E, STM-1, DTS-1, BIR-1, G-2, W-2) using this method of sample preparation are in close agreement with published values (<5% relative for many elements including the REE) and indicate the accuracy of this method.

This method was then applied to the granitic complexes of the Midcontinent Rift (MCR), commonly termed granophyres. The granophyres consist of basal diorite and monzodiorite and progress upward to quartz monzodiorite, granodiorite, and granite (Kennedy, 200). This study focused on the petrogenesis of four of these comnplexes: the Greenwood Lake, Misquah Hills, Eagle Mountain, and Pine Mountain granophyres.

The four granophyre complexes addressed in this study have U-Pb zircon ages that fall into two distinct groups. The older granophyres include the Misquah Hills and Greenwood Lake complexes and have been termed "early stage granophyres". The younger granites include the Eagle Mountain aned Pine Mountain bodies and have been termed "main stage granophyres". The granophyres are grouped according to the stage identified by Miller and Vervoort (1996). Isotopic and trace element data suggest both fractional crystallization (FC) and assimilation fractional crystallization (AFC) processes were involved in the evolution of the granophyres. The main stage granophyres have low Nb/Y ratios, a signature of contamination by crustal materials, whereas the Nb/Y ratios of the early stage granophyres are positively correlated with La/Sm. The main stage granophyres are also isotopically enriched (Epsilon Nd -3 to -8) suggesting assimilation whereas the early stage granophyres are not enriched (Epsilon Nd 0- to -2) suggesting that if assimilation did occur, older, enriched crustal materials were not assimilated. These data support the model of rift evolution as presented by Vervoort and Green (1997).