Stratigraphy, petrology and geochemistry of the Brook Street volcanics and Drumduan Group, eastern Nelson, New Zealand

Abstract

Poorly exposed strata of acid to basic volcanic rocks, and sediments of the Brook Street Volcanics and the Drumduan Group, form the fault bounded Brook Street terrane in eastern Nelson. Predominantly marine sequences of redeposited ash tuffs, fine-grained tuffaceous mudstone, siltstone, volcanoclastic sandstone and turbidites are characteristics of the Grampian and Groom Creek Formations of the Brook Street Volcanics. Volcanic breccias, tuffs and lavas are found mainly in the Kaka Formation. Atomodesma shell fragments and Peruvispira discovered in the Grampian Formation are consistent with a Permian age for the Brook Street Volcanics. Rocks of Botanical Hill Formation of the Drumduan Group include fine-grained basalts, lithic-and crystal-rich mafic tuffs, and rarer felsic tuffs. Outcrops of marine sediments suggest continental derived basin fill deposits, and include lenses of red-coloured tuffaceous sandstone, mudstone and siltstone. Reworked tuffs, as well as green and pink tuffs which contain poorly sorted volcanic lithics and crystals in sheared red argillaceous matrix occur less commonly. Marybank Formation to the north consists of lava flows, pyroclastic breccias, tuff agglomerates, pumice beds, airfall and water-laid siliceous tuffs which display current bedded sequences and slump features. Clast-supported pyroclastic beds of poorly sorted pumice clasts, lithics and crystals are characteristic of ignimbrite deposits. The rocks of the Marybank Formation south of Hira Fault, consist of poorly bedded dark-grey breccia, interbedded grey to black mudstone, siltstone and sandstone. Plant macrofossils in the the Marybank Formation suggest a Jurassic age. The Delaware Shear Zone is a major tectonic break, and separates the southeast steeply dipping beds of the Brook Street terrane into two groups. The shear zone consists of a series of northeast striking faults, and between them are schistose and tectonised sediments from adjacent units of the Brook Street terrane which form the youngest Wakapuaka Phyllonite Formation. The lithologies of the Brook Street terrane suggest accumulation of volcanic products in an island arc type environment. The Brook Street Volcanics are predominantly marine island arc deposits, but subaerially emplaced pyroclastic rocks indicate a largely emergent arc in the Permian. Widespread pyroclastic tuff agglomerates, breccias, and shallow water tuffaceous sediments in the Drumduan Group suggest active volcanism near a continental margin. Plant macrofossils support a largely terrestrial environment in the Jurassic. The Brook Street Volcanics consist mainly of basaltic rocks. Dark-green clinopyroxene is the dominant phenocryst found in coarse-grained breccias, tuffs and porphyritic basalts. Basaltic rocks with an exceptionally high proportion of ferromagnesian minerals suggest an ankaramitic affinity. Olivine phenocrysts are replaced by chlorite but primary hornblende occurs in basaltic breccia together with diopsidic clinopyroxene, plagioclase and opaques. Plagioclase is the predominant phenocryst phase in basalts, andesites, dacites and rhyolites of the Drumduan Group. Altered pleochroic biotite phenocrysts are found in dacites, but euhedral magnetite crystals with distinct octahedral lamellae of hematite are abundant in green and pink tuffs. Acidic rocks in the Mary bank Formation display textures reminiscent of banded rhyolite, and pyroclastic deposits. Textures in the volcanic rocks of the Brook Street terrane include vitrophyric, variolitic and holocrystalline. Petrographic evidence, such as the increasing abundance of silica with decreasing age of the units from east to west supports a single evolutionary sequence. Metamorphic assemblages of the Brook Street terrane suggest low grade regional metamorphism from prehnite,pumpellyite facies to lawsonite-albite-chlorite facies and lower greenschist facies. Secondary minerals are prehnite, pumpellyite, actinolite, chlorite, albite, quartz, sphene, epidote and lawsonite. The metamorphic assemblages of the Drumduan Group correspond to paragenesis: clinozoisite + actinolite + chlorite + quartz to the right of reaction (2) of Winkler (1979). The steep gradient displayed by the metamorphic path of Drumduan assemblages indicates that the phases are very sensitive to changes in pressures and temperatures. The paragenesis: prehnite + pumpellyite + chlorite + quartz is comparable to the prehnite-pumpellyite facies of the Brook Street Volcanics and the Drumduan Group. These phases occupy a narrow field between reactions (5) and (6) in the pressure vs temperature diagram of Winkler (1979). Prehnite, lawsonite and clinozoisite are key minerals in the Drumduan Group. The distribution of these minerals corresponds to an increase in grade, or increasing depth of burial (or pressure) from the northwest to the southeast. Prehnite, lawsonite and clinozoisite are phases which indicate different temperature and pressure of metamorphism. The occurrence of these minerals in individual faulted blocks suggest possible allochthonous terranes within the Drumduan Group. The closure of the marginal sea as a result of continuous subduction, juxtaposed the Drumduan Group with the Brook Street Volcanics, resulting in the prehnite-pumpellyite facies metamorphic event. Rapid deposition of volcanic deposits in a locally subsiding basin resulted in the development of high pressure/low temperature lawsonite bearing assemblages. Intrusive bodies, subsequent uplift and erosion may have helped to increase the temperature, and decrease the pressure of Drumduan rocks, thus resulting to the breakdown of lawsonite to low-Fe epidote. Major and trace element geochemistry of the Drumduan Group volcanic rocks shows a range from basaltic to rhyolitic composition. The data also display some bimodality. The high silica Drumduan rocks have features characteristics of calc-alkaline suite. The Brook Street Volcanics are lower in silica, and show a trend of moderate iron enrichment. In the Ti, Zr and Y discriminant diagrams of Pearce and Cann (l973), the basalts of the Brook Street Volcanics consistently plot in the island arc tholeiite field. Low TiO2, Y, Zr contents and Zr/Y, Ti/Y, Nb/Zr ratios of the Brook Street Volcanics are geochemical traits typical of tholeiitic island arc basalts. Plots of basalts and basaltic andesites of the Drumduan Group show a wider scatter. However, similar Ti/Y ratios with the Brook Street Volcanics suggest a similar source. Linear plots exhibited by both suites indicate that the low silica rocks of the Drumduan Group are the more fractionated equivalent of the Brook Street Volcanics. Two possible reconstructions of the Brook Street terrane are presented. The models suggest two periods of igneous activity which are consistent with a Permian tholeiitic island arc, and later calc-alkaline volcanism in the Jurassic. The Permian Brook Street Volcanics have widespread subaerial pyroclastic deposits which are indicative of a largely emergent island arc. An outcrop of high-Mg andesite suggests a period of rifting, possibly associated with the formation of a back arc basin. Prismatic Atomodesma shell fragments, and calcareous tuffaceous sediments support the presence of a marginal sea behind the Brook Street arc. There is no evidence of Triassic volcanism in the Brook Street terrane of eastern Nelson. However, the data suggest a change in the mode of volcanism probably in the Triassic because the Jurassic Drumduan Group has affinity with a continental volcanic arc, and is superimposed on the earlier island arc setting. The continuous subduction from Permian to Jurassic telescoped the facies of the marginal basin behind the Permian Brook Street Volcanics arc, and caused the centre of volcanism to shift to a region that had a quasi-continental structure. The volcanic products of the Brook Street terrane changed in character from purely island arc assemblages to transitional calc-alkaline eruptives. In the Jurassic, the Drumduan lavas were probably affected by upper crustal contamination, thus altering their primary chemical characteristics.