Research

Find out about my PhD research on the igneous and experimental petrology of Grenada, Lesser Antilles, or read about my MSci project on the metamorphic petrology of two terrains in south-west Greenland.


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PhD OVERVIEW

DIFFERENTIATION OF MANTLE-DERIVED MAGMAS BENEATH GRENADA, LESSER ANTILLES

Supervisors: Prof. Jon Blundy, Dr Elena Melekhova, Prof. Richard Arculus (ANU)

BACKGROUND

Grenada geology map
Position of Grenada in the Lesser Antilles island arc (click to enlarge)

Volcanism in the Lesser Antilles is the surface expression of the subduction of the North American tectonic plate beneath the Caribbean plate, and Grenada is the southernmost volcanic centre of the island arc chain. The island itself has five volcanic centres dating from ~10Ma with the last eruption taking place ~1000 years ago. Products of post-Miocene volcanism range from dacites and andesites to silica-undersaturated basalts, including the most primitive lavas of the Lesser Antilles. Like many of the Lesser Antilles volcanoes, Grenada also boasts an impressive suite of cumulate xenoliths; coarse-grained igneous rocks that formed deep in the magma storage system beneath the island before being eviscerated during violent eruptions.

The ultimate goal of the project is to replicate the various cumulate assemblages by conducting equilibrium experiments on parental basalts under a range of volatile (H2O, CO2) contents and pressures, and elucidate the conditions of magma storage and differentiation beneath Grenada.

CUMULATE XENOLITHS

The igneous cumulate suite of Grenada is notable for its petrologic diversity, ranging in composition from wehrlite and hornblendite to gabbro. In addition, Grenada is the only island in the Lesser Antilles to produce mantle-derived peridotite xenoliths.

A field campaign was conducted in March 2011 to collect further cumulate and scoria samples from Grenada and the Grenadines.

The results of major (electron microprobe) and trace element (in-situ LA-ICP-MS) analyses of mineral phases from plutonic xenoliths, along with complementary lava phenocrysts, are detailed in:

– Stamper CC, Blundy JD, Arculus RJ (in prep) Trace elements of volcanic rocks and plutonic xenoliths from Grenada, Lesser Antilles.

– Stamper CC, Blundy JD, Arculus RJ (2014) Petrology of volcanic rocks and plutonic xenoliths from Grenada, Lesser Antilles. Journal of Petrology, 55: 1353-1387. 167:954 DOI: 10.1093/petrology/egu027

EXPERIMENTAL PETROLOGY

The high-MgO basalts on Grenada are considered to be parental to the entire volcanic suite and therefore make an ideal template on which to base any experimental work.

Equilibrium experiments on a primitive basalts were carried out at conditions pertaining from the lower crust to the surface using piston cylinder, TZM and 1 atm apparata. Using scanning electron microscopy (SEM), electron microprobe and secondary ionisation mass spectrometry (SIMS) analyses, the effect of varying P, T XH2O, XCO2 and fO2 on mineral assemblages is be quantified and used to delineate phase relations.

whole capsule
Back scattered electron (BSE) image of a sample capsule after a high pressure experiment in a piston cylinder. Capsule width = 8 mm.

– Stamper CC, Melekhova E, Blundy JD, Arculus RJ, Humphreys MCS, Brooker RA (2014) Oxidised phase relations of a primitive basalt from Grenada, Lesser Antilles. Contributions to Mineralogy and Petrology. 167:954 DOI: 10.1007/s00410-013-0954-6. Read summary here.


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MSci PROJECT

COMPARISON OF METAMORPHIC DEVELOPMENT IN THE TRE BRØDRE AND FAERINGHAVN TERRANES, SOUTH-WEST GREENLAND

Supervisor: Dr. John Schumacher

BACKGROUND

The currently accepted model for the evolution of the Archaean craton of south-west Greenland is one of terrane assembly1,2. On this basis, the area south of the capital Nuuk is subdivided into allochthonous domains, each representing a parcel of crust hypothesized to have developed separately prior to accretion into one landmass at ~2.7Ga. However, recent fieldwork has identified marked similarities between terranes3 and has questioned the validity of some of the boundaries between them4.

locations
Geological map of sample localities. Dashed line shows approximate position of terrane boundary. Map compiled from Geological Society of Greenland 1:10,000 Map, sheet: Buksefjordan 63 V.1 Nord.

METHODOLOGY

The adjacent Tre Brødre and Færingehavn terranes are separated by virtue of significant disparities in their metamorphic histories1,2. In order to test this assertion, a combination of textural analysis and thermodynamic modelling of metamorphic reactions (using Perple_X) was employed to estimate peak metamorphic conditions and reconstruct the P-T path of both terranes.

RESULTS

Samples from both terranes comprise cordierite-orthoamphibole-garnet gneisses, derived from hydrothermally altered volcaniclastic protoliths, and exhibit spectacular reaction textures.

micrographs
Reaction textures in Færingehavn [F] and Tre Brødre [TB] rocks.
(Bt) biotite; (Cor) corundum; (Crd) cordierite; (Ged) gedrite; (Kyn) kyanite; (Plag) plagioclase; (Qz) quartz; (Sil) sillimanite.
(a) Crd rim around Sil grain in Ged [TB]
(b) SEM BSE image of Plag and Crd corona around Sil grain in a Ged and Qz matrix [TB]
(c) Aluminous enclave of Cor, St and Kyn in Crd in a Ged and Bt matrix [F]
(d) Fracturing in Grt [F]
(e) Kyn polymorphing to Sil with a Crd rim. St and Kyn show epitaxial growth [TB].

Mutually coexisting orthoamphiboles and the absence of pyroxene in amphibolites constrain peak temperatures to 600-700ºC. Modelling results indicate that both localities experienced a clockwise P-T path, attaining peak conditions of 4±0.5kbar and 625±50ºC. There is no evidence to suggest that the rocks ever experienced pressures >4.5 kbar or reached granulite-facies conditions.

pt
Summary of modelled reactions and P-T trajectories for Tre Brødre and Færingehavn. (An) anorthite; (Anth) anthophyllite; (Cor) corundum; (Crd) cordierite; (Fe-Sp) Hercynite; (Ged) gedrite; (Grss) grossular; (Grt) garnet; (Hcrd) hydrous cordierite; (Qz) quartz; (Sil) sillimanite; (St) staurolite

CONCLUSIONS

The finding that Tre Brødre and Færingehavn underwent extremely similar metamorphic development casts a doubt on their classification as separate terranes, and on the wider applications of the terrane assembly model.

REFERENCES

  1. Friend CRL, Nutman AP, McGregor VR (1987) Late-Archaean tectonics in the Færingehavn-Tre Brødre area, south of Buksefjorden, southern West Greenland. J. Geol. Soc. Lond. 144, 369-376.
  2. Friend CRL, Nutman AP, McGregor VR (1988) Late Archaean terrane accretion in the Godthab region, southern West Greenland. Nature. 335, 535-538.
  3. Keulen N, Schersten A, Schumacher JC, Næraa T, Windley BF (2009) Geological observations in the southern West Greenland basement from Ameralik to Frederikshab Isblink in 2008. Geological Survey of Denmark and Greenland Bulletin. 17, 49-52.
  4. Windley BF, Garde AA (2009) Arc-generated blocks with crustal sections in the North Atlantic craton of West Greenland: Crustal growth in the Archean with modern analogues. Earth Sci. Rev. 93, 1-30.