Document Type Master's Dissertation Author Van Tonder, Danel Magda email@example.com URN etd-11042010-213246 Document Title Petrology and geochemistry of the granitoids of the Halfway House Dome Degree MSc Department Geology Supervisor
Advisor Name Title Dr H Mouri Supervisor Keywords
- Petrology and geochemistry
- Kaapvaal Craton
- Halfway House Dome
Date 2010-09-02 Availability restricted Abstract
Growing support for analogies drawn between present-day plate tectonic processes and geotectonic processes active during Archaean times has led to re-evaluations of important Archaean terrains world-wide in the light of growing support for the notion that plate tectonic processes were active in the Archaean. Archaean cratons are considered to consist typically of three main rock associations, i.e. greenstone belts, tonalite-trondhjemite-granodiorite (termed “TTG suite”) and gneisses and calc-alkaline K-rich granitoids. The latter is often referred to as granodioritic-granitic-monzogranitic (GGM). Archaean TTG associations are the main components of the Archaean continental crust generated between 4000Ma and 2500Ma whereas the calc-alkaline GGM or high K-granodiorite suites dominate large parts of Archaean cratons, generally post-date the TTGs (2800 to 2500Ma) and are fed by vertical dykes cutting through the TTGs. TTG suites have a distinctly different geochemical signature when compared to GGM suites.
The Kaapvaal Craton represents one of only a few areas where pristine mid-Archaean rocks have been preserved. The Archaean Kaapvaal Craton formation took place in two distinct periods, i.e an initial shield-forming stage, which spanned from ~3700 Ma to 3100 Ma, followed by the stage of accretion of continental fragments and stabilisation spanning the period between 3100 Ma to 2600 Ma. The central Kaapvaal Craton, like most plutonic domains within Archaean cratons, is dominated by granitoid rocks of the tonalitetrondhjemite- granodiorite (TTG) series. Of the Archaean rocks in the central domain of the Kaapvaal Craton, those in the Halfway House Dome is the best exposed. Consequently this granitoid-greenstone terrane is well suited for studies of Archaean crustal evolution and the study area is therefore delineated as the window of Archaean granitoids occurring between Johannesburg and Pretoria, South Africa. The main objectives of the present work is to present an integrated field, geochemical and petrological study aimed at constraining the petrogenetic relationship between the constituent granitoids of the Halfway House Dome, to illustrate the existence of a TTG suite in the centre of Kaapvaal Craton, propose possible tectonic setting and processes involved in the generation of these rocks, discuss the TTG models for the Archaean crustal evolution of the central Kaapvaal Craton and to draw comparisons between the Halfway House Dome and other known TTG occurrences in South Africa and elsewhere in the world. The granitoids of the Halfway House Dome represent the final product of a multitude of physical and chemical processes, which occurred in the past history of the continental crust. The chemistry and mineralogy of these rocks therefore reflects the parental magma from which they were derived as well as the physical and chemical conditions under which the magma developed and solidified. Therefore the study of the granitoids of the Halfway House Dome is very important in understanding the evolution of the continental crust and to provide information on conditions under which the central part of the Kaapvaal Craton formed.
The thesis presents a geological map, a set of new petrographical and mineral composition data, together with whole rock major, trace and rare earth element data for a suite of granitoid rocks from the 3.3-3.0Ga Halfway House Dome, central Kaapvaal Craton. A model for the formation of the Archaean TTG suites on the Halfway House Dome is presented which reconciles the most important geochemical similarities and differences between Halfway House Dome granitoids and other world occurrences of TTGs.
Based on the microscopic and geochemical investigation the Halfway House Dome granitoids could be subdivided into three main suites, ie. a Tonalite Gneiss suite (TG) around the southern boundary, a Granodiorite-to-Adamellite Gneiss suite (GAG) across the northern part, and a Granodiorite-to-granitic suite (GG) occurring between the TG and GAG suites. The Halfway House Dome is dominantly I-type, peraluminous rocks with tonalites (TG and tonalite to trondhjemite gneiss GAG suites) falling in the metaluminous field. TTGs of the HHD are high-K calc-alkaline to calc-alkaline and are dominant high silica rocks (~70wt%), aluminous (Al2O3 >15wt%) with low Yb (<1ppm), high La/Yb ratios (>30), high Na2O/K2O (>1), and have Na2O contents of between 3wt% and 5wt%, comparable to that of the average TTG. The Halfway House Dome tonalities (TG suite) have higher Al2O3, Sr, Na2O/K2O, Mg#, Ni, Cr and LILE contents compared to the more calc-alkaline granitoids (GG suite and granodiorite-to-adamellite gneiss of the GAG suite), which are typically richer in HREE (lower REE fractionation), Y and show a negative Sr and Eu anomaly. Other characteristic features of the HHD TTG’s include HFSE depletion and distinct enrichment of fluid sensitive elements such as Pb. The strongly fractionated REE pattern, high (La/Yb)N ratio and depletion in HREE (Yb) of the Halfway House Dome TTGs are characteristics shared with modern adakites.The TG suite show a trend characteristic of high-Mg diorites (MgO>6 wt% with SiO,sub>2 ~50wt%) such as those described from the Pilbara Craton, Superior Province as well as high- Mg adakites. The high MgO, Ni and Cr contents of typical high-Mg diorites are considered to favor melting of subducted oceanic slab rather than underplated basalt, in which case felsic magmas are prevented from coming into contact with mantle peridotite. High-Mg diorites are considered to be relatively scarce (<5% of all Archaean TTGs) with very few, if any, pre-3 000Ma TTG suites showing this trend. The scarcity of high-Mg diorites suggests that the conditions for formation were not met in all Archaean terranes. The recognition of high-Mg diorites on the Halfway House Dome is therefore noteworthy as it signifies that the conditions necessary for high-Mg diorites formation were met during the formation of the TG suite, which is present in a limited area along the southern edge of the Halfway House Dome. The TG suite most probably formed through melting of a subducted oceanic slab with the melt interacting with mantle peridotite during its accent through a thin mantle wedge.
The remaining HHD granitoids (GAG granodiorite-to-adamellite gneiss and GG) most probably formed through the remelting of a TTG protolith, which has a subducted slab and mantle wedge signature (similar to the TG suite). This is proven by the presence of restite phases and the geochemistry of the GAG granodiorite-to-adamellite gneiss and GG suite approximating or imaging that of the TTG protolith, most probably the TG suite. Disequilibrium textures, such as reaction rims and the presence of Ca-rich plagioclase cores in the GAG granodiorite-to-adamellite gneiss and GG suites are indicative of a restite phase. The negative Eu anomaly and the absence of a positive Sr anomaly for the GAG granodiorite-to-adamellite gneiss and GG suites furthermore reflect the presence of plagioclase in the source.
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Please cite as follows:
Van Tonder, DM 2010, Petrology and geochemistry of the granitoids of the Halfway House Dome, MSc dissertation, University of Pretoria, Pretoria, viewed yymmdd < http://upetd.up.ac.za/thesis/available/etd-11042010-213246/ >
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