Geochemistry of igneous rocks from the southern coast belt plutonic complex, southwestern British Columbia
Creators: name: Cui, Yao
Publisher: name: University of British Columbia
Year: 2008

description: Source materials and magmatic differentiation have been constraint by Nd, Sr and Pb isotopic analyses and major, trace and rare earth element data for calc-alkaline plutonic and volcanic rocks collected along a 180 km transect from Vancouver to Anderson Lake in the southern Coast Plutonic Complex (CPC), southwestern British Columbia. The southern CPC comprises mainly Jurassic to Tertiary plutonic rocks, ranging in composition from granite, tonalite, quartz diorite, diorite to gabbro, with an average composition of quartz diorite. Subordinate Permian to Quaternary metamorphic and volcanic rocks make up the remainder of the southern CPC. Nd, Sr, and Pb isotopic data suggest that plutonic and volcanic rocks from the southern CPC consist mainly of mantle-derived materials with little or no involvement of old continental crust. Plutonic and volcanic rocks have initial ENd of +5.0 to +9.8 and initial 87Sr/86Sr ratios of 0.70244 to 0.70483. Values of Pb isotopic ratios for the suite range from 18.30 to 19.03 for 206Pb/204Pb, 15.43 to 15.67 for 207pbp/204Pb and 37.64 to 38.30 for 208pb/204pb. 208pb/204Pb rises slightly with decreasing age for a subset of volcanic rocks. Examined against position along the transect, the isotopic compositions are uniform along the transect from Vancouver to Pemberton, except for a subtle rise in ENd values for plutonic rocks. Northeast of Pemberton, a rise in initial 87Sr/86Sr towards the NE end is defined by volcanic rocks. 207Pb/204Pb ratios for plutonic rocks drop at Pemberton and then rise toward the NE margin of the southern Coast Belt. The isotopic variations with distance probably suggest either an important tectonic boundary near Pemberton (e.g., west margin of the Wrangellia terrane) or differences in source region characteristics. Based on the Nd and Pb isotopic data three different origins are proposed for igneous rocks of the southern CPC: 1) partial melting of mantle wedge, 2) anatexis of juvenile terranes (e.g., Wrangellia), and 3) combinations of 1 and 2. Geochemical characteristics and magmatic differentiation of plutonic rocks from this transect in the southern CPC are further examined by mineralogy and chemical analysis.Based on petrography the calc-alkaline plutons are divided into two compositional suites:(1) a mafic suite comprising tonalite, quartz diorite, diorite and gabbro with abundant hornblende and little or no K-feldspar and (2) a felsic suite of plutons comprising granite,granodiorite, tonalite and quartz diorite with K-feldspar as an important phase and rarehornblende. Two fractionation hypotheses are proposed to explain the chemical variations for these two suites of plutonic rocks: sorting of plagioclase + hornblende + biotite ± titanite ± apatite for the mafic suite and sorting of plagioclase + biotite + K-feldspar titanite ± apatite for the felsic suite. Pearce element-ratios were employed to discriminate the mineral assemblages and to test these two hypotheses on samples from this transect. The tests demonstrate that these two hypotheses can explain the chemical variations. The importance of intercepts on Pearce element ratio diagrams was explored in testing the comagmatic hypothesis, distinguishing source regions and discriminating magma batches. The variation pattern of intercepts against distance is similar to that of Nd, Sr and Pb isotopic compositions and incompatible element ratios for plutonic rocks from this transect area. This similarity suggests that the intercepts can be used to discriminate source regions for plutonic rocks. Therefore, two distinct source regions for plutonic rocks along the transect is confirmed by intercepts: one SW of Pemberton and another NE of Pemberton. Different source regions may reflect a tectonic boundary between Wrangellia terrane SW of Pemberton and other inner terranes NE of Pemberton, or abroad geochemical difference between NE and SW of Pemberton, an important conclusion also suggested by isotopic systematics. Based on intercepts of Pearce element-ratio diagrams, 6 magma batches are also recognized for this suite of plutons across the southern Coast Belt. Magma batch grouping is important in further discussion of the chemical affinity of the source materials for the plutonic rocks.