Miocene Low-K Dacites and Trondhjemites of Fiji

TLDR: High-Si dacites comprise one mode in each of two bimodal volcanic rock suites in Fiji as mentioned in this paper, and their compositions are similar to those of 20 to 40% partial melts of basalt at P H 2 O = 5 kb.

Abstract: High-Si dacites comprise one mode in each of two bimodal volcanic rock suites in Fiji. Both suites are low-K throughout but differ in level of iron-enrichment and light REE-enrichment. Sparsely-phyric dacite lava and tephra cover 10 2 -10 3 km 2 in each suite, apparently having erupted in shallow marine environments. Tephra predominates. Typical phenocrysts are plagioclase >>quartz >>Fe-Ti oxides and augite; hypersthene and hornblende are rare and biotite is absent. Most samples have >72% SiO 2 , 2 O 3 , and 2 O. Normatively, most have 6% Or and An x 100/(Ab + An) ratios of 20 over a wide range in Ab/Qz ratios; thus they cross the feldspar-quartz cotectic at a high angle. Their compositions are similar to those of 20 to 40% partial melts of basalt at P H 2 O = 5 kb. Tonal ite-trondhjemite and gabbro plutons also form a bimodal suite. Trondhjemite and dacite mineralogy and compositions are analogous although trondhjemites contain primary hornblende. REE patterns vary considerably in the three otherwise similar suites, ranging from light REE-depletion with La e.f. =8, to middle REE-enrichment with Nd e.f. = 360, to middle-REE depletion with Dy e . f = 7 (Fig. 5). The latter characterizes all trondhjemites analyzed and is compatible with fusion of lower Fijian crust.