Petrology, geochemistry and igneous and metamorphic mineralogy of low-grade metamorphosed basalts of the Torlesse terrane, South Island, New Zealand.

Author:

Pringle, Ian James.

Year:

Project type:

Advisers:

Abstract:

Field relations and petrography of low-grade metamorphosed basaltic rocks (metabasites) and associated lithologies from numerous localities in the Carboniferous to Jurassic Torlesse terrane and marginal Haast Schist terrane of North Otago and Canterbury are described.
Pillowed and massive metabasites (spilitized basalt, dolerite, porphyrite, and rare gabbro) together with other lithologies of the volcanogenic association (limestone, chert, volcanic breccia, tuff and red and green argillite) and their metamorphosed equivalents are mapped as discrete metabasite units. The largest, the Dansey Metavolcanic Formation (North Otago) crops out for over 28 kilometres along strike and has a maximum thickness of 760 metres.
Contacts between metabasites and greywackes are commonly unfaulted. Metagreywacke 'dykes', continuous and compositionally similar with underlying greywackes intrude basal pillowed feldspar metaporphyrite in the Waianakarua River metabasite unit.
At some localities in North Canterbury, isolated outcrops of volcanogenic sequences are parts of melanges (e.g. Cheviot area and Esk Head melange) or blocks in tectonic slides (e.g. Cavendish Hills in North Canterbury).
Major and/or trace element contents of 58 metabasites are presented together with data from Torlesse metabasites from Red Rock Point (Wellington) and Te Anau Assemblage metabasites from Taieri Reef (South Otago). 30 of the analysed samples are from the Dansey Metavolcanic Formation. Chemical analyses of zones in pillow structures and trace element comparisons between mineralogically different metamorphic vein-host rock pairs suggest that the elements: Ti, Zr, Y, Ni, and Pare relatively unaffected by secondary alteration. Discrimination plots incorporating the above, relatively 'immobile' elements [i.e. (Ti-Zr-Y), (Ti-Zr), (Ti-Zr/P2O5) and (Ni-Y)] indicate that the analysed metabasites include both tholeiitic and alkalic basalts. These diagrams also indicate that prior to secondary alteration, analysed Torlesse metabasites were compositionally similar to either: ocean-floor basalts, or basalts erupted within plates as seamounts or oceanic islands. Both types occur within the larger metabasite units.
Diagrams incorporating relatively 'mobile' oxides and elements (e.g. normative diagrams, AFM, ACF diagrams and plots including the elements; Sr, Rb, and Ba) are of limited use in magma-type discrimination of the Torlesse metabasites. However, metabasites with tholeiitic basalt affinities generally contain higher Cu, lower Zn and less variable Pb contents than metabasites with alkalic basalt affinities. Also, Miyashiro plots (FeO*/MgO vs Ti0 2 and FeO*/MgO vs FeO*) of unpillowed, massive metabasalts and metadolerites corroborate magma-type conclusions from 'immobile' element data. High FeO*/MgO ratios of pillowed and sedimentary metabasites is attributable to more pronounced secondary alteration of these samples rather than to magmatic differentiation.
Si02 , Na2O and K2O contents of samples with tholeiitic ocean floor basalt affinities indicate that these metabasites have undergone moderate amounts of pre-metamorphic weathering. Combined, pre-metamorphic weathering and metamorphism have resulted in 3.5- 4.0 wt % SiO2 loss and considerable mobilization of both Na2O and K2O for many analysed metabasites.
194 igneous clinopyroxene analyses, together with analyses of primary amphibole, plagioclase, Fe-Ti oxide, olivine and apatite from Torlesse metabasite units are presented. The relict phases indicate a spectrum of basalt compositions from alkalic to tholeiitic types. Titansalites in a metabasalt from the Lady Barker Range contain up to 7.15 wt % Ti02 and 10.21 wt % Al2O3 and are among the most Ti- and Al-rich terrestrial igneous clinopyroxenes recorded. Clinopyroxenes in another metabasalt from the same unit have variable Ca:Mg:Fe ratios which are similar to pyroxene trends in the classic tholeiitic Skaergaard Intrusion. Clinopyroxenes in metabasites from other Torlesse localities have compositions which are intermediate between these extremes. The spectrum of Ca:Mg:Fe ratios in clinopyroxenes from metabasites of the Dansey Metavolcanic Formation is closely comparable to that of oceanic island basalt suites (e.g. Hawaiian Islands).
Relict clinopyroxene data from Taieri Reef, South Otago (Te Anau Assemblage) are similar to average values of clinopyroxenes from island arc basalts.
For most metabasites the magmatic affinities indicated by both whole rock geochemistry and clinopyroxene data are in agreement. Sector zoned clinopyroxenes, however, give ambiguous results on conventional discrimination diagrams. Combined, the data indicate that the ratio of alkalic to tholeiitic metabasites in metabasite units within Carboniferous and Atomodesma fossil zones is lower than in Triassic fossil zones. Also, the basal portions of the Lady Barker Range metabasite unit and the Dansey Metavolcanic Formation are dominated by tholeiitic metabasites. These units are capped by metabasites with alkalic affinities.
These relationships, together with the lithological content of many of the units, conform with recent models of the structure and evolution of small, isolated, oceanic volcanoes which grew in areas of rapid sediment accumulation.

The metamorphic transition from pumpellyite-actinolite facies to chlorite zone greenschist facies for metabasite lithologies was studied in the Dansey Metavolcanic Formation (North Otago). Analyses of 37 pumpellyites, 63 actinolitic amphiboles, 25 riebeckite-arfvedsonites and riebeckitic actinolites, 81 chlorites, 96 epidotes and numerous analyses of sphene, phengite, stilpnomelane, albite and hematite are discussed in terms of host rock composition and textures and metamorphic mineral assemblage.
Pumpellyites include Fe-, Al- and Mg-rich varieties. On thin section scale pumpellyite compositions are remarkably heterogeneous. Iron-rich epidote is an ubiquitous phase in metabasites. Compositions of Ps25-36 are common, although in some rocks iron-rich epidote cores are rimmed by iron-poor epidote (Ps16-21). Epidote and pumpellyite heterogeneity reflect a variety of pumpellyite-consuming, epidote producing reactions within different microdomains of the same rock.
Mg:Fe ratios of colourless-pale green actinolite vary sympathetically with those of coexisting chlorite. However, within a single thin section, actinolites with a single cleavage parallel to rock foliation have higher. Mg:Fe ratios and SiO2 and CaO contents and lower Al2O3 , Ti02 and MnO contents than actinolites with two [110] cleavages. The differences suggest that the former were formed at slightly higher metamorphic grade. Blue amphiboles are minor phases in several metabasites. They are invariably mantled by actinolite and are considered to have formed during a pre-metamorphic episode of submarine alteration.
Chlorites are predominantly ripidolites and brunsvigites. Some contain significant MnO (up to 4.32 wt %) and Cr2O3 (up to 2.42 wt %).
Stilpnomelanes include Mn-, Fe 3+- and Fe 2+-rich varieties. Comparison with compositions of the phase in other metamorphic terranes indicates that Al/[Fe + Mg + Mn] ratios increase with increasing metamorphic grade.
Primary Fe-Ti oxides in some Dansey Pass metabasites have been altered to pseudobrookite-hematite assemblages and later to sphene (± hematite) pseudoroorphs.
The disappearance of relict clinopyroxene in massive metabasites approximately corresponds with the pumpellyite-clinozoisite isograd for metagreywacke lithologies. However, metamorphic mineral transitions between the pumpellyite-actinolite and the chlorite zone greenschist facies are less sharply defined in metabasites than in metagreywackes. Rather, metabasite metamorphic mineral assemblages are closely related to host rock textures. Pillowed and sedimentary metabasites are dominated by chlorite-epidote-actinolite assemblages whereas massive metabasites (flows or conformable sills) are less permeable to metamorphic solutions and commonly contain pumpellyite-bearing assemblages.

In southern New Zealand, minerals of the axinite group are widespread in vein assemblages in regionally metamorphosed rocks of the prehnite-pumpellyite, pumpellyite-actinolite and chlorite zone greenschist facies. Fe, Mg-axinites, approaching endmember ferroaxinite in composition, along with quartz and often prehnite, pumpellyite, iron-rich epidote and chlorite fill veins in spilitized volcanic and greywacke lithologies. Tinzenite and more commonly manganaxinites occur in quartz veins in nearby ferruginous and manganiferous cherts. Average and representative analyses of seventeen vein axinites are presented as well as analyses of four porphyroblastic ferroan manganaxinites which occur as rock-forming minerals at widely separated localities.
Compositional variability in published axinite analyses along with those of this study can be attributed partly to formation temperatures. A low temperature miscibility gap may exist in the axinite group. Tinzenite or manganaxinite and ferroaxinite are stable in low-grade metamorphic rocks of appropriate compositions, whereas ferroan manganaxinites and manganoan ferroaxinites occur in some pegmatites, skarns and regionally metamorphosed rocks which equilibrated at more elevated temperatures.
Analyses of tourmaline intermediate in composition between end-members dravite and schorl are presented for five rocks from the Dansey Metavolcanic Formation. Compositions of the mineral in regionally metamorphosed rocks appears to be more influenced by bulk rock composition than by metamorphic grade.
For a wide range of bulk compositions in biotite and garnet zone greenschist facies and amphibolite facies rocks of southern New Zealand, tourmaline is the only observed borosilicate phase. At metamorphic conditions typical of these grades, axinite minerals would be restricted to relatively Ca-rich lithologies by a reaction of the form: 3 ferroaxinite + 5 chlorite + 2 albite + 5 quartz = 2 tourmaline + 4 epidote + 2 actinolite + 5 water.

Named Localities:

Thesis description:

440 p. : ill. ; 30 cm.

Department:

OU geology Identifier:

1980Pringle

Author last name:

OURArchive handle:

OURArchive access level:

Location (WKT, WGS84):

MULTIPOLYGON (((170.530128882000099 -44.575959743999931,170.450169194000068 -44.549829536999937,170.451019386000098 -44.53381646899993,170.542506007000043 -44.560469833999946,170.54991662000009 -44.562624906999986,170.545426342000042 -44.580951723999931,170.530128882000099 -44.575959743999931)),((170.916995042000053 -44.734875362999958,170.934770160000085 -44.735068464999983,170.933680325000068 -44.760740305999946,170.916281232000074 -44.760409873999947,170.916995042000053 -44.734875362999958)),((170.631522583000105 -45.261412632999964,170.631611411000108 -45.258565412999985,170.632663371000035 -45.225289471999986,170.66872262600009 -45.231303577999938,170.681830123000054 -45.262400033999938,170.650422814000081 -45.261785055999951,170.631522583000105 -45.261412632999964)))

Files

http://download.otagogeology.org.nz/temp/Abstracts/1980Pringle.pdf

Collection

Citation

Pringle, Ian James., “Petrology, geochemistry and igneous and metamorphic mineralogy of low-grade metamorphosed basalts of the Torlesse terrane, South Island, New Zealand.,” Otago Geology Theses, accessed February 7, 2025, https://theses.otagogeology.org.nz/items/show/136.

Output Formats