Geology of the Round Hill area, western Southland.
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Separated from Fiordland by the Waiau basin is the north south trending Longwood Range. A short distance from the Southland coast on the western side of the Longwood Range is situated Round Hill.
The dense bush covered predominantly gabbroic rocks of the Longwood Range and the similarly covered promontory of gabbroic rocks at Round Hill are connected by an isthmus of native bush through which runs a narrow body of intruded quartz diorite. The quartz diorite probably represents the material formed in the thermal metamorphic reaction between gabbro and an injected granite which outcrops in a narrow zone between the two on the western side. This view is supported by the degree of alteration the quartz diorite have suffered and the unusual shape of the body.
To the west of Round Hill the volume of granite-granodiorite rocks intruded is more extensive. Small patches of gabbro have been isolated from the main gabbroic body by these plutons which have a limited border zone of hybrid rocks in the contact between the two. Other hybrid rocks occur among the granodiorite with no evidence of gabbro in the immediate region which suggests the gabbro has been almost entirely assimilated. There is a gradation between the hybrid rocks and granodiorite indicating possibly much of the granodiorite was formed in reaction between granite and gabbro. Quartz diorite also graded into granodiorite so it is likely we have the same sort of thing occurring here.
Granite and not granodiorite is usually in contact with gabbro with small leucocratic veinlets intrusing for a small distance into both rocks. Xenolith bearing granite often occurs close to the gabbro.
The gabbro of Round Hill and the Longwood Range where exposed. has a weathered surface in which irregular layers of segregated mafic and non mafic minerals a few centimetres thick are evident. Occasional foliation in granite is the only primary structure in the acid rocks.
The dense bush covered predominantly gabbroic rocks of the Longwood Range and the similarly covered promontory of gabbroic rocks at Round Hill are connected by an isthmus of native bush through which runs a narrow body of intruded quartz diorite. The quartz diorite probably represents the material formed in the thermal metamorphic reaction between gabbro and an injected granite which outcrops in a narrow zone between the two on the western side. This view is supported by the degree of alteration the quartz diorite have suffered and the unusual shape of the body.
To the west of Round Hill the volume of granite-granodiorite rocks intruded is more extensive. Small patches of gabbro have been isolated from the main gabbroic body by these plutons which have a limited border zone of hybrid rocks in the contact between the two. Other hybrid rocks occur among the granodiorite with no evidence of gabbro in the immediate region which suggests the gabbro has been almost entirely assimilated. There is a gradation between the hybrid rocks and granodiorite indicating possibly much of the granodiorite was formed in reaction between granite and gabbro. Quartz diorite also graded into granodiorite so it is likely we have the same sort of thing occurring here.
Granite and not granodiorite is usually in contact with gabbro with small leucocratic veinlets intrusing for a small distance into both rocks. Xenolith bearing granite often occurs close to the gabbro.
The gabbro of Round Hill and the Longwood Range where exposed. has a weathered surface in which irregular layers of segregated mafic and non mafic minerals a few centimetres thick are evident. Occasional foliation in granite is the only primary structure in the acid rocks.
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50 leaves. Photos. Map (folded in pocket); 30 cm.
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1980Rombouts
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POLYGON ((167.763299649540329 -46.277988182696419,167.813139494588029 -46.280266313805733,167.810972656922047 -46.310046438396348,167.76106284152371 -46.308216964425547,167.763299649540329 -46.277988182696419))
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Rombouts, MJ, “Geology of the Round Hill area, western Southland.,” Otago Geology Theses, accessed May 15, 2025, https://theses.otagogeology.org.nz/items/show/138.