Rodney Grapes

Uplift and exhumation of Alpine Schist, Southern Alps, New Zealand: Thermobarometric constraints

Abstract Thermobarometric estimates of early (D1) and late (D2) metamorphic recrystallisation of Alpine Schist, Southern Alps of New Zealand, define three polychronous piezothermic arrays: (1) 4.3–7.5 kbar and 260–370°C for chlorite and biotite‐albite zone metagreywacke‐semischist; (2) 5.2–9.2 kbar and 415–620°C for garnet and oligoclase zone schist, and (3) 5.4–6.6 kbar and 490–540°C for K‐feldspar zone schist near the Alpine Fault. The metamorphic arrays are used to develop a simple one‐dimensional model to explain metamorphism and uplift of the Alpine Schist in terms of a P‐T‐t(time) loop. Evidence from retrograde albite (together with phengitic mica and K‐feldspar) in oligoclase zone schist implies an early (possibly Mesozoic) schist uplift of 7–8 km corresponding to a temperature and pressure decrease of c. 100°C and 2.0–2.3 kbar. Thermobarometric data from K‐feldspar zone schist suggests a further, possibly Late Cretaceous, uplift of 3–6 km giving a total Mesozoic uplift of the Alpine Schist of betw...

Shallow dolerite intrusion and phreatic eruption in the Allan Hills region, Antarctica

Abstract Shallow intrusion of dolerite sills into porous and permeable Permian-Triassic strata in the Allan Hills region resulted in steam pressures which exceeded that of the roof rocks causing fracturing and fluidisation of the sediments and the formation of volcanic mudflows and explosion breccias. Shallow dolerite intrusion is suggested by curved and domed sills which were intruded parallel to surfaces of equal vertical rock pressure and magmatic pressure (compensation surfaces) so that they became gently curved to mirror the overlying topography. En echelon sills formed at shallow depths where folding and bedding-plane slip in the sediments was possible. A wedgeshaped clastic sill associated with a clastic dyke extending from it have a matrix similar to that of the mudflow breccias. The clastic sill is considered to be a preserved extension space which formed in front of an intruding dolerite sill. Fracturing of the roof of the extension space resulted in a fluidised mixture of mud, steam, and rock f...

Rupturing of the Awatere Fault during the 1848 October 16 Marlborough earthquake, New Zealand: Historical and present day evidence

Abstract Evidence from newspaper reports, diaries and journals, related first‐hand information, an 1854 survey map of the lower part of the Awatere Valley, and reports by geologists between 1856 and 1890, indicates that surface rupturing occurred on the Awatere Fault during the 1848 October 16 earthquake and not on the Wairau Fault as previously inferred. The rupture was initially described as a “fissure”, “crack”, and later as a “rent”, and although it extended for c. 105 km (from the coast to Barefell Pass), it was not termed a fault because displacement of the land surface or strata across the rupture could not be determined. The coincidence of the 1848 earthquake “rent” and the Awatere Fault was first demonstrated by Alexander McKay in 1885. Present day evidence of the 1848 Awatere Fault rupture is indicated by a depression between 0.6–1.5 m wide and c. 0.3 m deep that has the appearance of an infilled fissure similar to that described in early reports. The smallest and freshest displacements along th...

Aenigmatite, sodic pyroxene, arfvedsonite and associated minerals in syenites from Morotu, Sakhalin

AbstractAenigmatite, sodic pyroxene and arfvedsonite occur as interstitial minerals in metaluminous to weakly peralkaline syenite patches in alkali dolerite, Morotu, Sakhalin. Aenigmatite is zoned from Ca, Al, Fe3+-rich cores to Ti, Na, Mn, Si-rich rims reflecting the main substitutions Fe2+Ti4+⇌Fe3+, NaSi⇌CaAl and Mn2+⇌Fe2+. Aenigmatite replaces aegirine and ilmenite supporting the existence of a ‘no-oxide’ field in

Geochemistry and origin of piemontite-bearing and associated manganiferous schists from Arrow Junction, western Otago, New Zealand

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Manganiferous schists and their origin, Hidaka Mountains, Hokkaido, Japan

— T space. In one case aenigmatite has apparently formed by reaction between ilmenite and arfvedsonite. Titanian aegirine (up to 3.0 wt% TiO2) and Fe-chlorite may replace aenigmatite. Sodic pyroxene occurs as zoned crystals with cores of aegirine-augite rimmed by aegirine and in turn by pale green aegirine containing 93 mol% NaFe3+Si2O6. Additional substitution of the type NaAl⇌CaFe2+ is indicated by significant amounts (up to 6 mol%) of NaAlSi2O6. Arfvedsonite is zoned with rims enriched in Na, Fe and depleted in Ca which parallels the variation of these elements in the sodic pyroxenes.The high peralkalinity of the residual liquid from which the mafic phases formed resulted from the early crystallization of microperthite (which makes up the bulk of the syenites) leading to an increase in the Na2O/(Na2O+K2O) and (Na2O+K2O)/Al2O3 ratios of the remaining interstitial liquid which is also enriched in Ti, Fe, and Mn. Bulk composition of the melt,

Neoblastic jadeitic pyroxene in Franciscan metagreywackes from Pacheco Pass, central Diablo Range, California, and implications for the inferred metamorphic P‐T trajectory

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