Geology of the Kaikoura Area - GNS Science

iotite granite, locally with megacrystic K-feldspar, andfine-grained muscovite-biotite granite (Dkg). At MtMantell, biotite granodiorite and tonalite (Dkt)predominate (Stewart 1974). U-Pb dating of the KarameaSuite beyond the map area gave crystallisation ages of388–358 Ma (Muir & others 1994, 1996).Median BatholithThe rocks separating the Eastern and Western provincesinclude a range of mafic, intermediate and felsic plutonicintrusions, some extrusive equivalent rocks and severalsignificant sedimentary units in and to the north of themap area. The Median Tectonic Zone (MTZ, Bradshaw1993) was defined to include these diverse and, in places,structurally dismembered rocks. Mortimer & others (1999)recognised that the MTZ is more than 90% plutonic inorigin and that much of the deformation was superimposedin the Cenozoic. Moreover the plutonic rocks extendbeyond the western limit of the MTZ into parts of theWestern Province and are viewed as part of an originallycontiguous batholith, the Median Batholith (Fig. 4,Mortimer & others 1999; Mortimer 2004). The batholith isdominated by I-type plutonic rocks ranging from ultramaficthrough to felsic compositions (Mortimer & others 1999).In the Kaikoura map area the Median Batholith is obliquelytruncated by the Alpine Fault to the southeast, and thewestern contact is obscured under the Murchison Basin.Northwards from the Alpine Fault the Median Batholithbecomes increasingly obscured by a cover of late Cenozoicrocks, dominated by the Moutere Gravel, although it reemergesnortheast of Nelson city. In Northwest Nelsonthe batholith intrudes the Takaka terrane (Rattenbury &others 1998). In the east the batholith is separated from theBrook Street terrane by the Delaware-Speargrass FaultZone (Johnston & others 1987; Johnston 1990; Rattenbury& others 1998). The Median Batholith includes the RotoroaComplex, Tasman Intrusives and Separation Point Suite.The latter two intrusive units are separated by the FlaxmoreFault. Adjacent sedimentary and volcanic rocks, rangingfrom subgreenschist to amphibolite facies metamorphicgrade, are volumetrically minor and some have been derivedfrom extrusive equivalents or erosion of the MedianBatholith intrusive rocks.Late Triassic to Late Jurassic igneous rocks andassociated sedimentary rocksThe Tasman Intrusives group several Late Triassic to LateJurassic plutons in the southeast of the MoutereDepression (Johnston 1990). The Buller Diorite (Tab), nearLake Rotoiti, consists of medium-grained, commonlyaltered, diorite to tonalite (Fig. 15a) with sparse dikes offine-grained diorite and quartz-rich pegmatites. The BullerDiorite characteristically contains plagioclase (oligoclase/andesine) and hornblende with minor interstitial quartz andaccessory minerals. Southeast, towards the Delaware-Speargrass Fault Zone, it becomes progressively foliatedwith the development of alternating white to greyish-white,feldspar-rich and dark grey to black, biotite and hornblenderichlayers. Within the Delaware-Speargrass Fault Zonethe diorite grades into the foliated Rotoiti Gneiss (Tar)composed of quartz, plagioclase (oligoclase), K-feldspar,biotite, muscovite and porphyroblastic garnet. Radiometricdating of the Buller Diorite yielded Late Triassic ages of225 Ma (K-Ar, Johnston 1990) and 228 Ma (U-Pb zircon,Kimbrough & others 1994). Other rock types within theDelaware-Speargrass Fault Zone include altered coarsetofine-grained gabbro and minor pyroxenite of theSeventeen Gabbro (Jas) of Mesozoic age (Johnston 1990).The One Mile Gabbronorite (Jao) consists of diorite,tonalite and hornblende gabbronorite that are commonlyaltered (Tulloch & others 1999). The pluton intrudes theRainy River Conglomerate in the east and is bounded bythe Flaxmore Fault in the west. U-Pb dating of zirconsindicates a 147 Ma (Late Jurassic) intrusive age (Kimbrough& others 1994).Big Bush Andesite (Jeb) of the Teetotal Group (Johnston1990) forms an 800 m wide strip in the upper Rainy Riverand consists of grey to greenish-grey, fine- to mediumgrainedandesite and overlying andesitic breccia. Theandesite is largely massive but flows, up to 2 m thick, withdark, fine-grained margins are locally recognisable. It ischaracterised by andesine phenocrysts with a dominanttrachytic texture. The breccia is predominantly unstratifiedalthough locally fragments are weakly aligned. The brecciagrades eastwards, and probably stratigraphically upwards,into the c. 2000 m thick Rainy River Conglomerate (Jer).Within the conglomerate are rare sills or flows of porphyriticBig Bush Andesite. The lower 200 m of the Rainy RiverConglomerate is dominated by andesitic breccia with layersof subrounded to rounded andesitic pebbles. This brecciagrades upwards into very poorly sorted, greenish-greyconglomerate with a variety of clasts, up to 0.5 m across, ina fine- to coarse-grained grey or greenish-grey sandstonematrix (Fig. 15b). The clasts include andesite, felsicplutonics, basalt, and altered sandstone and siltstone. Inplaces the clasts are dominated by diorite eroded from theunderlying Buller Diorite (Tulloch & others 1999). TheRainy River Conglomerate contains rare, very thicksandstone beds locally with coal seams up to 0.4 m thick,and minor siltstone. The formation has undergone zeolitefacies metamorphism and the coal is of high-volatilebituminous rank.Clasts within the Rainy River Conglomerate are sourcedfrom the Median Batholith and the Brook Street terrane,the closest terrane of the Eastern Province. No clasts fromthe Western Province have been identified. The ages ofthe Big Bush Andesite and Rainy River Conglomerate arenot known, although they are constrained by theunderlying Buller Diorite and the intruding One MileGabbronorite (see above). U-Pb zircon dating of two quartzmonzonite clasts from the Rainy River Conglomerateyielded 273–290 Ma (Early Permian) and 176–185 Ma (EarlyJurassic) ages. A late Middle or Late Jurassic age isassigned to the Big Bush Andesite and Rainy RiverConglomerate (Tulloch & others 1999).14