Trace element zonation in granulite-facies garnet porphyroblasts, Ikertôq, Nagssugtoqidian Orogen, west Greenland

Geoff T. Nichols and Norman J. Pearson

Macquarie University, GEMOC, School of Earth Sciences, Sydney, NSW, 2109, Australia.

INTRODUCTION

Location and regional significance

Pelitic gneisses crop out for approximately one kilometre across strike in Amerdloq Fjord, Ikertôq region, south of Sisimiut (Holsteinsborg). They occur in the footwall, or on the southeastern side of a NNE trending structure mapped as a thrust (Bak et al. 1975; Grocott 1979). Recent detailed mapping (Hanmer et al. 1995) indicates that the NNW dipping thrust is younger, and defines the northwestern margin of, a shear belt characterised by straight gneisses and moderately steep mineral stretching lineations. The pelites are predominantly composed of garnet-sillimanite-orthoclase-phlogopite-ilmenite±quartz, and display a strong mineral elongation (plunging 50° towards 241°, defined by sillimanite, biotite, and weakly elongate garnet), and a well developed foliation (dipping 56° towards 337°). Garnet porphyroblasts are coarse grained and are typically 1-2 cm in diameter (eg, sample 415227). The gneisses contain numerous leucocratic veins subparallel to the well defined foliation, and are themselves partially boudinaged or necked, and in places folded. The veins interfinger with the gneissic layering, and their variable development appears to reflect layering heterogeneities.

The Ikertôq thrust is thought to juxtapose amphibolite facies gneisses, which occur to the southeast, with granulite facies rocks in the northwest. This study is aimed at defining metamorphic episodes, associating them with the deformational history of the thrust zone, and thereby determining the pressure-temperature-deformational history of this significant structural feature.

An extremely coarse grained xenoblastic garnet porphyroblast (415228) measuring ~11.5 cm by 7 cm, was sampled from a thin (0.5 m) leucocratic vein within the pelitic gneisses. The garnet contains numerous inclusions of acicular sillimanite, prismatic phlogopite, ilmenite, orthoclase and quartz. The coarser grained matrix adhering to margins of the garnet is composed of the same mineral assemblage as the inclusions. The garnet preserves smooth boundaries with the matrix, and large laths of biotite interfinger with the garnet edges. The garnet and biotite laths are truncated by zones composed of acicular sillimanite, and orthoclase. In folia which anastomose the garnet, coarse laths of brown biotite are intimately intergrown with sillimanite and elongate ilmenite. Similar microtextures are preserved in pelite 415227 from adjacent gneisses: garnets are xenoblastic, weakly elongate parallel to the lineation, have rounded cuspate boundaries with the matrix, and are anastomosed by coarse-grained sillimanite-rich folia.

RESULTS

X-ray and microprobe data

X-ray maps recording the distribution of Mg, Fe, Ca and Mn within the large garnet porphyroblast (415228) were produced using a Cameca SX50 electron microprobe, with an accelerating voltage of 15 KeV, a beam current of 50 nA and a dwell time of 60 ms per spot (each X-ray map image is composed of 1024 x 1024 spots). These maps were combined with quantified microprobe analytical traverses, and indicate that the large garnet porphyroblast preserves chemical zonation not recorded by the smaller garnets in adjacent gneisses. The core of the large garnet porphyroblast is enriched in Mn, Ca, and slightly in Fe, compared with the outer regions. The inner region, which makes up approximately 7.5% of the volume of the garnet, is significantly depleted in Mg (Fig. 1).

Figure 1. - A montage of four separately measured images displaying the distribution of Mg-X-rays in a coarse garnet porphyroblast from sample 415228. The darker central region has less pyrope and more almandine, than the outer brighter region. Dark regions on the circumference are epoxy, and the lower left-hand map does not reach the garnet edge. The total area of the maps is 7.5 x 6 cm.

These chemical variations suggest that the inner region preserves compositional information acquired at lower pressures and or temperatures than conditions during the growth of the outer garnet regions. Garnet 415228 has the following compositional variations: the core regions preserve Alm80Prp14Gr4Sp2, intermediate regions are Alm79Prp16Gr4Sp1 and outer regions are Alm75Prp21Gr4. In contrast garnets from an adjacent pelite (415227) are essentially homogeneous with Alm75Prp20Gr4Sp1, close to the composition preserved in the outer areas of the larger garnet.

Trace element data

In order to further understand and define the conditions experienced during the growth of this garnet, trace element compositions were determined using the LAM-ICPMS (Laser Ablation Microprobe-Inductively Coupled Plasma Mass Spectrometer). The LAM is used as a solid sampling device for the ICPMS and thus allows in-situ trace element analysis of minerals with sub-ppm detection limits for most elements (Norman et al. in press). The LAM consists of a frequency quadrupled Nd-YAG laser, operating at 266 nm (UV) and an optics system that focuses the laser beam through a petrographic microscope onto the sample. The sample is housed in a sealed cell and the ablated material is carried by Ar directly into the torch of the ICP. A video camera attached to the microscope allows the ablation process to be viewed. LAM operating conditions for the analysis of the garnet produced a spot size of ~30 µm at a power of ~1 mJ/pulse. The raw data acquired from the garnet were quantified using the CaO content as an internal standard (determined by electron microprobe) and the NIST 610 glass as an external reference standard using the methods detailed in Norman et al. (in press).

Trace element traverses of two garnets, from samples 415228 and 415227, were positioned to cross regions preserving major element zoning, or to cross the centre of garnets. Eighteen trace elements were determined for each analysis point along the garnet traverses. The elements display two distinct groupings, a feature common to both garnets.

One group includes the elements V, Co, Zn, Ga, Sm, Sc, Nd, Eu and Hf, characterised by symmetric, gradational zonation profiles. V, Sm, Nd and Eu show an increase from core to rim, whereas Sc, Hf, Zn and Co produce reverse patterns.

The second group of elements includes Ni, Y, Zr, Gd, Dy, Ho, Er, Yb and Lu, all of which show consistent patterns across the garnet and generally increase from rim to core. However, unlike the first group, the zoning is not gradational, but instead is marked by zones of relative enrichment and depletion.

DISCUSSION

Figure 2 shows chondrite normalized trace element plots for the two garnets, selectively displaying a single, typical, core and rim analysis for each garnet. A number of features are apparent in this diagram:

1. The LREE abundances are similar for core and rim in both garnets but in contrast there is a marked depletion, relative to core, in HREE in the rims.

2. Both garnets preserve this HREE zonation, whereas garnet 415228 additionally preserves major element zoning.

3. The trace element compositions of the cores of the two garnets are similar, although 415228 has a more pronounced negative Eu anomaly, suggesting that the core grew in equilibrium with plagioclase (although no plagioclase was observed in the groundmass).

Figure 2. Chondrite normalized plot displaying core and rim analyses from garnets 415227 and 415228. Both garnets preserve trace element zoning, although only 415228 records major element zonation. The HREEs display the largest variations between garnet cores and rims.

Possible explanations for these features include:

1. Trace elements diffuse more slowly than major elements (Mg-Fe) in garnet. If this is true then trace element zonation can be used to model the P-T evolution of garnet-bearing rocks.

2. The central regions of the garnets in the two samples grew under the same compositional conditions, but clearly different to those that produced the rims. Equilibration with a leucosome saturated with a REE-rich accessory phase such as xenotime, during the later garnet-growth stage may have caused HREE depletion in the rims.

Research is currently underway to test these possibilities and includes modelling the diffusion profiles of various trace elements combined with mineral-melt phase equilibria.

Acknowledgements

GTN thanks the DLC for funding participation in the inaugural Nagssugtoqidian Orogeny field trip, as well as the California Institute of Technology for funding travel to Copenhagen. This research would not have been possible without the facilities at Macquarie University: we additionally thank Ashwini Sharma for ICPMS operational expertise, and Marc Norman for ongoing developmental work of the LAM-ICPMS system. Tom Bradley is thanked for producing the excellent large thin-sections of garnet 415228. Leo Kriegsman provided constructive suggestions which improved our presentation.

REFERENCES

Bak, J., Korstgård, J.A. & SØrensen, K. 1975. A major shear zone within the Nagssugtoqidian of West Greenland. Tectonophysics, 27: 191-209.

Grocott, J. 1979. Shape fabrics and superimposed simple shear strain in a Precambrian shear belt, W. Greenland. J. Geol. Soc. Lond. 136:471-488.

Hanmer, S., Mengel, F., Connelly, J. & van Gool, J. 1995. A re-examination of crustal-scale shear zones and synkinematic mafic dykes in the Nagssugtoqidian orogen, SW Greenland: a summary. Proceedings - DLC Workshop "Nagssugtoqidian Geology 1995", 21-28.

Norman, M.D., Pearson, N.J., Sharma, A. & Griffin, W.L., (in press) Quantitative analysis of trace elements in geological materials by laser ablation ICPMS: instrumental operating conditions and calibration values of NIST glasses. Geostandards Newsletter.

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