Geochemistry and Nd-isotope systematics of sediments from the Dun mountain ophiolite: II - terrigenous sediments

Full text: Two distinct Early Permian clastic sedimentary associations occur within the Dun Mountain Ophiolite Belt (DMOB), New Zealand. These include: (a) an early deep marine graded sandstone - turbidite argillite (TA) association intercalated with ophiolitic lower pillow lavas and upper sheeted flows (DMOB stage 1 and 2 (back-arc basin) basalts); and (b) a coarse matrix-supported, lithic-dominated sandstone + ophiolitic rudite assemblage associated with stage 3 (infant arc - forearc) magmas. The geochemistry of the sediments constrains relations between the tectonic setting of ophiolite emplacement, contiguous Palaeozoic terranes and palaeo-Pacific morphotectonic elements (e.g., Gondwana rim and island arcs). Stage 1 pillow lavas are overlain by red mudstones (20 m thick), passing upwards into green (redeposited) hyaloclastite (<20 m), and unfossiliferous graded sandstones and red and green TA (<70 m), with variable (but diminishing) metalliferous contributions that unequivocally link the TA to stage 1 (back-arc) lava. A (<50 m) thick band of ungraded Atomodesmid-bearing grey TA is intercalated between lower and upper lavas. Following initial quiet suspension (and chemical) sedimentation, turbidite deposition took place in a shallowing deep-marine environment, with andesitic-dacitic TA compositions reflecting volcanism during pre- to syn-rift back-arc basin evolution. The TA show systematic trends of decreasing Eu/Eu* and ε Nd(T) (+2, +1 and 0.5 in red, green and grey TA, respectively) with increasing ε REE, La/Y, Th/Sc (0.1-0.8) and SiO2 (53-67 wt%) Their trace element contents resemble evolved island arc rocks (e.g. Brook Street Volcanics (BSV) andesite-rhyodacite with similar (La Yb)N and Eu/Eu*). But low positive ε Nd(T) values imply source components less depleted than rocks from island arcs or the ophiolite itself (ε Nd(T)=+6 to +9). Th abundances and Th/Sc ratios (up to 2.0) are higher in the TA than in island arc and ophiolitic suites, similar to those of continental arcs. (Moreover, the oldest rocks in the BSV are ∼265 Ma, some 15 m.y. younger than the youngest (stage 3) DMOB magmas.) Antipathetic ε Nd(T) Th/Sc, SiO2 correlations may reflect mixing between isotopically depleted island arc components and enriched continental crustal materials. The narrow range of positive ε Nd(T) values for the TA resembles values for calc-alkaline magmas from the Southern Volcanic Zone of the Andes (ε Nd(T)=+2; Th/Sc=0.03) and suggests a predominant young differentiated continental arc source rather than old upper crust, but less dissected than the quartzofeldspathic plutonic provenance for the Torlesse (ε Nd(T)<-2). The slightly higher ε Nd(T) values (and lower Zr Nb etc.) for red and green TA suggest a minor 'hydrothermal' contribution from stage 1 basalts. The TA overlap a far wider compositional field for sediments from the Caples terrane. Exclusively terrigenous sandstones + ophiolitic rudites comprise a younger bimodal sedimentary suite of differing tectonic affinity. The coarse sandstones are rich in clinopyroxene of ophiolitic affinity, with compositions (e.g. Alz-Ti distributions) like pyroxenes from DMOB stage 3 magmas, but overlapping with pyroxenes from primitive arc basalts. They also contain significant amphibole, not present in BSV tholeiites, but abundant in ophiolitic plagiogranites. Massive poorly stratified, matrix-supported rudites (monomictpolymictic ophiolitic breccias) comprise discontinuous beds up to 10 m thick and contain abundant lithic fragments of stage 3 (infant arc - forearc) rocks, ranging from silicic plagiogranite to rare ultramafics. The sandstones possess 50-58 wt% SiO2, with slight LREE enrichment ([CeN/YbN]=2.5-2.8) (less than for the TA with [CeN/YbN] = 4.1-4.9), and REE and incompatible element patterns closely resembling BSV island arc tholeiites. Th/Sc ratios are low (<0.3), implying a more mafic source than for the TA. The sandstones do not show the pronounced LREE depletion of DMOB stage 3 magmas ([CeN/YbN]=0.53-1.65). However, ophiolitic breccias, with their higher SiO2 (∼69 wt %), low Eu/Eu*(∼0.72). [CeN/YbN]∼1.56, ThiSc (0.10.2), and higher, flatter HREE (TbN/YbN∼1.0) and convex upward LREE-MREE (i.e. Iower Ce/Nd, La/Sm), are transitional between the sandstones and plagiogranites. Importantly, the mafic sandstones and silicic breccias show high positive initial ratios (εNd(T)=+5 to +8), more akin to the high positive values for BSV eruptives. This implies mixing between varying amounts of young (mafic and silicic) mantle-derived source materials. εNd(T) values exceed the high positive values for Caples sandstones (εNd(T)=+3.6-+5.5). Crustal residence (mean provenance) ages are lower in the DMOB sandstones and rudites (TCR=0.46-0.84 Ga) than for the TA (1.0-1.2 Ga). In contrast to Maitai, Caples and Torlesse sediments, little continental input is permitted by Nd isotopic data for these rocks. Predominantly continental arc sources are also precluded by the lower SiO2 contents of the sandstones compared to the TA (unlike Torlesse sandstones). During nascent subduction, bimodal magmatism at infant arc volcanic centres in an extensional forearc setting, separated from continental clastic sources, shed turbidite sands and mass-flow deposits into proximal fault-bound basins. Copyright (1999) Geological Society of Australia