12/22/2023 0 Comments Cultures northland mapsThe melting parameters and compositions of the sources involved are modelled using trace element ratios and multi-element patterns, and three components are characterized: (1) fertile peridotite with a Pb-isotope composition similar to Pacific mid-ocean ridge basalt (2) eclogite domains with a HIMU-like isotope composition dispersed within the fertile peridotite (3) slightly depleted subduction-metasomatized peridotitic lithospheric mantle (containing c. Whole-rock multi-element patterns show that the larger centres have prominent positive Sr anomalies and lack Kanomalies, whereas the smaller centres have prominent negative K anomalies and lack Sr anomalies. Three end-members are observed in Pb isotope space, indicating that distinct mantle source components are involved in the petrogenesis of the magmas. Fractional crystallization has had a limited effect in many of the centres, but high-pressure clinopyroxene crystallization may have occurred in others. Whole-rock compositions range from subalkaline basalt in the larger centres, through alkali basalt to nephelinite in the smallest centres. These show the large compositional variations between volcanoes as well as through single eruptive sequences. We present data for 12 centres in the volcanic field. The Quaternary Auckland Volcanic Field (AVF) in northern New Zealand is a basaltic field of 51 small-volume volcanic centres, and is one of the best-sampled examples of a monogenetic volcanic field. High-resolution sampling in monogenetic fields has the potential to reveal fine-scale heterogeneity of the mantle, a feature that may be overwhelmed by larger fluxes of magma, or missed by under-sampling. The presence of larger-scale defects has been problematic for tunnelling, including the presence of low shear strength clay seams along ECBF bedding planes, and bedding planes intersecting tunnel crowns. The ECBF sandstone, where uncemented, is much weaker and is highly permeable, while the presence of smectite clay may be problematic for tunnelling if it disaggregates, forming a ‘sticky spoil’. Specific issues identified include lenses of the heavily zeolitized Parnell Volcaniclastic Conglomerate, and marked strength contrasts within the weaker ECBF siltstone and sandstone. Samples were also extracted for laboratory analysis of elastic wave velocities, intact strength parameters and thin section analysis of microtexture. Here, the geological strength index (GSI) and rock mass rating (RMR) were applied to outcrop in coastal cliff exposures located on Auckland’s North Shore, to evaluate the engineering geological properties of the ECBF. Therefore, excavations in the region can encounter highly variable rock mass. Auckland is mainly underlain by Miocene-age East Coast Bays Formation (ECBF), a highly heterogeneous flysch deposit, and is also affected by lava flows from the Late Quaternary Auckland Volcanic Field (AVF). Engineering geological characteristics of the rapidly growing Auckland urban area are of increasing interest due to ongoing and planned tunnels and excavations.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |