Geology of the Upper Eglinton area : status of the median tectonic line.
Williams, John G.
In the upper Eglinton area, the Eglinton Volcanics (Permian) are intruded by plutonic rocks mapped as the Mackay Intrusives. A sharp fault contact separating the Eglinton Volcanics from crystalline rocks of the Fiordland Complex, i.e. separating New Zealand's eastern and western provinces is no longer recognised in this area and rocks mapped as Darran Diorite by Wood (1962) are now mapped as Mackay Intrusives. The Eglinton Volcanics are mapped as two terranes separated by an elongate body of quartz-bearing diorite, the Mistake Diorite. The eastern (Plato) terrane , which consists largely or entirely of marine deposits, comprises a steeply dipping, east-younging sequence of volcanogenic sediments of basaltic to andesitic composition, intruded by abundant basaltic dikes. Four formations are defined: the basal (Gonder) formation consists of predominantly massive, course grained pyroclastic detritus and includes un Atomodesma limestone member; subsequent formations (Consolation, Divide and Fergus) are mainly of sand to silt grade rocks and include turbidites. Fossils from two new localities include brachiopods - Ambikella, Sulciplica transversa, Aperispirifer (?), Spiriferellina and bivalves – Atomodesma marwicki, Etheripecten and Conocardium (?). They confirm an Early to Middle Permian age for the Plato terrane. Rocks of the western (Largs) terrane are largely of andesitic to dacitic composition and predominantly volcaniclastic, although massive andesite bodies are mapped at either end of the terrane. Fossils, limestones and turbidites are unknown, but strongly hematitic rocks, restricted to minor occurrences in the Gonder Formation of the Plato terrane, are common. The terrane may be partly of terrestrial origin. Basaltic andesite and andesite dikes are fairly common but several distinctive dike lithologies found in the Plato terrane, e.g. augite porphyrite, Cr-diopside ankaramite and microdiorite, are absent in Largs rocks. The southern part of the terrane appears to be folded into a tight syncline, however the structure is not completely resolved and formal formations have not been established. Age of the Largs terrane and its relation to rocks of the Plato terrane remains unknown. It is herein assumed to be of Early to Middle Permian age, though a younger, e.g. Mesozoic origin, cannot be discounted. Eglinton rocks are shown to be of tholeiitic to calc-alkalic affinity and are considered to be the products of Lower Permian island arc volcanism which accumulated either as separate arcs (i.e. Plato and Largs), or as separate portions of the same arc, approximately 180-190 km above a descending lithospheric plate. Low-grade burial metamorphism occurred, mainly during Permian time and parts of the Largs terrane have been hornfelsed during subsequent Mackay intrusion. Aspects of regional metamorphism are also recognised and this can probably be regarded as a Mesozoic development. Three metamorphic zones are mapped: a prehnite-pumpellyite-epidote zone (I) where metamorphic grade is prehnite-pumpellyite transitional to pumpellyite - mactinolite- schist facies; an actinolite-epidote zone (II) occurring west of zone I with assemblages of the chlorite zone of the greenschist facies; a biotite-actinolite-epidote zone (III) restricted to the northwestern part of the Largs terrane where the terrane is intruded by gabbroic and dioritic Mackay rocks. Mineral assemblages in zone III are compatible with those of the greenschist facies as well as the albite-epidote and hornblende-hornfels facies of contact metamorphism. Textural modification is not widely developed in either terrane. The Mackay Intrusives are subdivided into six units. The Gunn Dolerit comprises two intrusive bodies in the northern Plato terrane, its age is unknown but it may be as old as Early Permian. Mistake Diorite is the name given to the elongate, relatively homogeneous body of medium to coarse grained quartz-bearing diorite which separates the Largs and Plato terranes. K-Ar ages of 208, 208, 186, 182 and 180 m.y. have been obtained from it and are interpreted as indicating a Triassic age of intrusion. The Hut Plutonic Suite, comprising mainly white leuco-granite, flanks part of the western margin of the Mistake Diorite and is inferred to be of similar age to the Mistake body. Hollyford, Nurse and Glade suites are mapping units and are not necessarily of genetic significance, i.e. together they comprise a genetically related series of granitoid to gabbroic rocks characterised by diverse composition on outcrop scale, medium grainsize, zoned plagioclase and lack of widespread deuteric alteration. Each suite comprises a mappable rock mass in which one lithology or a narrow range of lithologies predominates. Leuco-gabbronorite predominates in the Hollyford Gabbroic Suite, a unit mapped in the upper Hollyford area. Four K-Ar ages range from 130-136 m.y. (uppermost Jurassic to Cretaceous) and a cross-cutting trondhjemite dike gives concordant biotite and muscovite ages of 113 m.y. Rocks of the Nurse Plutonic Suite, mapped in the Nurse-Brandywine area, are granitoid to dioritic in composition. They are undated, but as rocks of identical composition and texture intrude Hollyford gabbronorites, a Cretaceous age seems likely. The Glade Plutonic Suite comprises texturally and compositionally diverse rocks in which dioritic types are dominant. K-Ar ages of 189 and 112 m.y. on a hornfelsed andesitic inclusion and recrystallized (?) leuco-granite respectively, suggest, in conjunction with field relations, a complex intrusive history. Microprobe analyses of rock forming minerals observed in a representative selection of plutonic and volcanic rocks are reported and discussed. In particular,Mg-Fe distribution coefficients (KD) have been calculated for coexisting pyroxenes from a series of gabbronorites across the Hollyford suite. KD changes systematically form east to west across the suite as the Fe:Mg ratio of the pyroxenes increases. This trend, and the concentration of olivine-bearing rocks in the eastern part of the suite, suggest that the gabbroic rocks may comprise a gravity-differentiated intrusion with its base towards the east. Several major north-to northeast-trending, high-angle faults are mapped. The Hollyford Fault marks the eastern margin of the Eglinton Volcanics and is considered to be a fundamental tectonic break. Skelmorlie Fault has offset Tertiary sediments of the Annick Group, perhaps by several kilometres in a sinistral strike-slip sense and/or by several hundred metres vertically (west side up). The Glade Fault, a newly described feature, has produced 5-6 km of apparent dextral offset in the Hut suite leuco-granite. The Eglinton Fault zone, mapped along part of the eastern margin of the Mistake Diorite, is characterised by gneissic mylonite containing sparse almandine garnet. A newly discovered sliver of Tertiary sediments faulted into the Plato terrane in Plato Creek, as well as Tertiary strata unconformably overlying Mackay and Eglinton rocks west of Mt Eglinton, are described briefly. Distinct differences in conglomerate provenance are noted within the Tertiary sequence near Mt Eglinton. Although a tectonic contact between New Zealand's eastern and western provinces is not recognised within the area mapped, it is concluded that the two provinces did not lie in their present relative position in the Permian and Early Mesozoic. If this conclusion is correct, then the present juxtaposition of the two provinces means that their mutual contact is, or was, tectonic. It is considered that the contact lies in approximately the same position as the median tectonic line as proposed by Landis and Coombs (1967) and that it has been subsequently obscured for most of its length by Mesozoic plutonism and Cenozoic tectonism and sedimentation. It is proposed that the term "median tectonic line" be retained for this contact between provinces of contrasted pre-Cenozoic geological evolution, notwithstanding the likelihood that the contact probably originated as a zone of tectonised rock rather than as a sharp break, as originally mapped.
Advisor: Landis, Charles
Degree Name: Doctor of Philosophy
Degree Discipline: Geology
Publisher: University of Otago
Research Type: Thesis