Stratigraphy, petrology, mineralogy, and metamorphism of mainly Triassic rocks, Hokonui Hills, Southland, New Zealand
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This thesis describes the stratigraphy, petrology, and mineralogy of about 260 sq. km of mainly Triassic rocks on the north limb of the Southland Syncline, Hokonui Hills, Southland. The thesis area consists of a 10km thick sequence of essentially undeformed, steeply dipping, mainly marine, volcanogenic sediments. The classical description of zeolite facies metamorphism (Coombs, 1954) was based on the Taringatura Hills, an area adjacent to the Hokonui Hills.
A lithologic map of the area and four detailed stratigraphic columns have been made. The study gives 20 bulk analyses of sandstones, siltstones, and tuffs. In addition, EMA analyses of heulandite group minerals, laumontite, stilbite, analcime, chlorite, celadonite, prehnite, pumpellyite, albitized plagioclase, and fresh pyroclastic plagioclase are reported. The alteration of heulandite to laumontite and of heulandite group minerals to analcime was investigated in laboratory experiments. A study was made of the relationship of composition to optical properties, cell dimensions, and thermal stability of some heulandite group minerals.
Numerous strike-slip faults and changes in strike indicate the area has undergone a compressive stress in a northwest-southeast direction, subsequent to the folding of the Southland Syncline.
Two units are recognized in the stratigraphically lowest sequence in the area: Ram Hill sandstones and the overlying Waimea siltstones. These units are unfossiliferous and are collectively referred to as the pre-Fairplace beds (1-1.2km thick). Overlying the pre-Fairplace beds is the North Range Group (3-3.5 km thick) which has been subdivided into six formations; Fairplace Formation, Gavenwood Tuffs, Crosshill Gully, North Peak Formation, and Stag Stream Siltstones, in order of younging. New fossil localities indicate that the Crosshill Gully Siltstones and most of the North Peak Formation are of Malakovian age (Lower Triassic, Scythian). The Stag Stream Siltstones is of Etalian age (Middle Triassic, Anisian).
The Taringatura Group (Middle and Upper Triassic), which overlies the North Range Group, has not been subdivided but at least one unit, the Bare Tuff Zone, is continuous across the area. The group thins from 5.4 km in the west to 1.8 km in the east. Rock units of the Taringatura Group are more fossiliferous, less traceable, and contain fewer plant fragments than those of the North Range Group.
Conglomerates are common throughout the sequence; some contain boulders up to 1.5m in diameter, the lensoidal nature of most conglomerate beds indicates they were deposited mainly in near-shore submarine channels, probably by traction currents.
Andesites are the predominant components of pre-Fairplace, North Range Group and Otapirian (Upper Triassic, Rhaetian) conglomerates. Rhyolites and dacites are the predominate volcanics in Kaihikuan (Middle Triassic, Ladinian) and Oretian (Upper Traissic, Carnian) conglomerates. Diorite, tonalite, and grandiorite pebbles are associated with the more basic volcanics whereas granites and adamellites are associated with the more, acid volcanics. Plutonic pebbles are most common in the Taringatura Group. This relationship, in addition to petrographic evidence, indicates that plutonic pebbles were derived from intrusive bodies underlying the volcanics.
Rudaceous-size material becomes generally finer grained from west to east in the Glenure Formation. Acid volcanic pebbles occur at a lower horizon in the eastern Hokonuis than in the western Hokonuis. The data indicate the source-area of the acid material is east of the source area of the more basic material.
Sandstones are usually massive, poorly sorted, and grading is not common. In many cases sands were probably transported by a grain flow process. Volcanic rock fragments or plagioclase predominates in the sandstones. Detrital quartz makes up <20% of the mode in all samples and in many cases is <5%.
The distribution of types of volcanic clasts in sandstones parallel those of conglomerate pebbles. In addition, Otamitan (Upper Triassic, Carnian) sandstones contain predominantly rhyolite and dacite fragments and Jurassic sandstones contain predominantly andesite fragments. The pattern is also verified by distribution of detrital clinopyroxene and Ti02 + Fe2o3 + FeO + MgO.
Sandstones with a color mottling on a scale of 3mm up to 1cm are widespread but are especially common in the Taringatura Group. The sandstones are frequently laumontite-rich but the mottled color is mainly due to the unequal distribution of chlorite cement.
Non-resistant siltstones are common throughout the sequence. Thick sequences of resistant siltstones occur mainly in the Taringatura Group. Resistant and non-resistant siltstones frequently contain animal trails and borings and traction current indicators, but grading is uncommon. The siltstones were probably deposited at similar depths to the sands but during times of minor tectonic activity. Many resistant siltstones contain albite and/or quartz cements.
Vitric and vitric-crystal tuffs are common throughout the sequence and have been altered mainly to heulandite, analcime, laumontite, and montmorillonite. The tuffs are believed to have been transported mainly by air and to have undergone little reworking. In several cases the deposits were prone to slumping. Graded tuffs are common in the Taringatura Group. Relict shards in tuffs of the Gavenwood Tuffs become progressively finer grained from west to east. Fresh pyroclastic feldspars in Oretian tuffs are more sodic than in Malakonian, Etalian and Otapirian tuffs.
Vein minerals in the area include laumontite, stilbite, analcime, heulandite group, albite, quartz, calcite, halloysite, and pumpellyite. Laumontite and stilbite are the most common types and occur mainly in pre-Oretian, Otapirian, and Jurassic rocks.
The distribution of secondary minerals is related to the bulk composition of the rocks. Secondary cements of prehnite, pumpellyite, laumontite and heulandite are restricted to sandstones containing predominantly andesitic clasts. High proportions of sphene and chlorite are also found in these samples. Albite and/or quartz cements are usually abundant in sandstones containing rhyolitic or dacitic clasts. Laumontite is abundant in Otapirian and Jurassic sandstones as well as in the North Range Group and pre-Fairplace beds. Prehnite is common in the North Range Group and occurs sporadically in Otapirian and Jurassic beds.
Plagioclase in fine sandstones ( < 0.2mm average grain size) is much less prone to albitization than in coarser sandstone from the same part of the sequence. Grains which are completely altered have a composition of Ab99.1 An0.9. Albitized portions of grains which are partially fresh have compositions significantly more calcic than completely albitized grains. There is a general tendency for increased albitization towards the base of the section if sandstones of similar grain-size are compared.
Laumontite occurs as a replacement of heulandite,detrital plagioclase and as a cement. Alteration of a heulanditized tuff to a laumontitized tuff is nearly an isochemical alteration except that MgO and Fe2O3 + FeO are significantly lower in the laumontitized tuff than in the heulanditized sample due to removal of phyllosilicates. The alteration has been controlled in part by jointing in the rocks.
Analcime pseudomorphs after heulandite are common and hence most analcime is believed to have been formed from heulandite group minerals. Analcime can be synthesized from heulandite group minerals at 100°c with solutions of NaOH (0.1m) and Na2CO3 (0.1M and 0.01M) in 3-week runs. Both increased pH and Na+ concentration favor the reaction. The Si/Al ratio of the analcime product is largely a function of the Si/Al ratio of the zeolite reactant. Analcime compositions in the Hokonuis are uniform throughout the sequence and appear to have equilibrated with quartz.
Heulandite group minerals in the Hokonuis typically replace volcanic glass or fills cavities in sandstones. They have a wide range of Si/Al ratios (3.0 to 4.3), are rich in Ca, have K/Na ratios '1 in most samples, and sometimes have appreciable Mg. Composition does not vary systematically with depth of burial. However, minerals with relatively high Si/Al ratios are particularly common in Oretian and Otamitan tuffs. Large crystals in cavities or glass vesicles of tuffs have lower Ca contents and higher Si/Al ratios than the crystals replacing glass shards. Composition of the heulandite group minerals is believed to have been controlled by the bulk composition of the glass.
Albite is a more common cement or cavity filling than quartz or K-feldspar. Albite replacement of heulandite has been documented in several samples.
Prehnite replaces plagioclase, fills cavities, and is inferred to have replaced heulandite in numerous samples. frequently with heulandite than with laumontite. Prehnite occurs more Two varieties are recognized: a "clear" type which contains 4.6 - 6.0 wt % total Fe as Fe2O3 and an inclusion riddled "spongy" type which contains 1.2 - 3.2 wt % total Fe as Fe2O3.
Pumpellyite usually occurs in the matrix but replaces plagioclase in some samples. Some of the pumpellyite is inferred to have replaced laumontite. The pumpellyite is optically negative and two optical orientations are recognized. The mineral contains up to 14 wt.% total Fe as Fe2O3.
Chlorites in the Hokonuis have wide variations in Al/Mg + Fe ratios (o.36- 0.56). Celadonites, which are most abundant in tuffs, have wide variations in Al/Fe ratios (0.51 - 2.80). Aluminous chlorite and aluminous celadonite co-exist in a tuff. EMA analyses are given for phases in tuffs which have compositions intermediate to chlorites and celadonites and may indicate an interlayered chlorite and celadonite.
Mixed-layer clay minerals, including both random and regular mixed-layer types, are not uncommon. Mixed layer montmorillonite-chlorite and montmorillonite-mica have been identified.
The geothermal gradient in the Hokonuis is estimated from mineral stability fields to be about 20°C/km. The mineral distribution pattern is interpreted in terms of variations in solution chemistry, initial rock composition, temperature, load pressure, and fluid pressure.
A general approach towards regional equilibrium is suggested by restricted zeolite mineralogy, absence of volcanic glass, and generally more advanced reconstitution· towards the base of the section, several examples of local disequilibrium are cited. However, several examples of local disequilibrium are cited.
Study of heulandite structural group. There is a full range of Si/Al ratios between heulandite and clinoptilolite end members. Ca, Na, and K can replace each other extensively, especially in clinoptilolites. Therefore, a new classification is proposed for heulandite group minerals based on Si/Al ratios. The subdivisions are made: heulandite (Si/Al < 4.00) and clinoptilolitG (Si/Al > 4.00). The dominant exchangeable cation in the unit cell is placed as a suffix to the subdivision e.g. heulandite-Ca.
If both loosely and tightly bound water are considered then + 0.6 H2O molecules are coordinated about divalent cations; and 3.2 + 0.5 molecules about monovalent cations in heulandite group minerals. The variation in density and refractive index of heulandite group minerals is largely a function of the variation in total water content.
Optical orientation of crystals mounted in Lakeside 70 is length fast if Si/Al < 3.52 and length slow if Si/Al E 3.57. Refractive index is strongly influenced by the types of cations present.
Comparison of calculated cell dimensions indicate (1) clinoptilolites typically have smaller a, c, and b parameters than heulandites (2) the area of the ac plane increases with increasing Al or divalent cation substitution; (3) b increases with increasing Mg substitution.
Unit cell dimensions are given for two different contracted phases, phase I and phase B of four dehydrated heulandites. For heulandite from Cape Blomidon, Nova Scotia, the spacing of the (020) reflection of the untreated mineral is 8.970A and for phase B is 8.30-8.35A. The phases appear at temperatures as low as 202°C~ 3°. For 11 heulandites and clinoptilolitesthe initial change to phase I occurs at f 213* to 312*C + 3* after heating for 2 hours and cooling for 1 hour.
Three types of thermal stability are recognized in the mineral group. Minerals with a sum of unit cell divalent cations equal to or greater than 1.87 contract after heating. Higher temperatures are required to contract samples with higher Si/Al ratios.
Composition of the heulandite structural group zeolites can be estimated using optical, cell dimension, and thermal stability data.
A lithologic map of the area and four detailed stratigraphic columns have been made. The study gives 20 bulk analyses of sandstones, siltstones, and tuffs. In addition, EMA analyses of heulandite group minerals, laumontite, stilbite, analcime, chlorite, celadonite, prehnite, pumpellyite, albitized plagioclase, and fresh pyroclastic plagioclase are reported. The alteration of heulandite to laumontite and of heulandite group minerals to analcime was investigated in laboratory experiments. A study was made of the relationship of composition to optical properties, cell dimensions, and thermal stability of some heulandite group minerals.
Numerous strike-slip faults and changes in strike indicate the area has undergone a compressive stress in a northwest-southeast direction, subsequent to the folding of the Southland Syncline.
Two units are recognized in the stratigraphically lowest sequence in the area: Ram Hill sandstones and the overlying Waimea siltstones. These units are unfossiliferous and are collectively referred to as the pre-Fairplace beds (1-1.2km thick). Overlying the pre-Fairplace beds is the North Range Group (3-3.5 km thick) which has been subdivided into six formations; Fairplace Formation, Gavenwood Tuffs, Crosshill Gully, North Peak Formation, and Stag Stream Siltstones, in order of younging. New fossil localities indicate that the Crosshill Gully Siltstones and most of the North Peak Formation are of Malakovian age (Lower Triassic, Scythian). The Stag Stream Siltstones is of Etalian age (Middle Triassic, Anisian).
The Taringatura Group (Middle and Upper Triassic), which overlies the North Range Group, has not been subdivided but at least one unit, the Bare Tuff Zone, is continuous across the area. The group thins from 5.4 km in the west to 1.8 km in the east. Rock units of the Taringatura Group are more fossiliferous, less traceable, and contain fewer plant fragments than those of the North Range Group.
Conglomerates are common throughout the sequence; some contain boulders up to 1.5m in diameter, the lensoidal nature of most conglomerate beds indicates they were deposited mainly in near-shore submarine channels, probably by traction currents.
Andesites are the predominant components of pre-Fairplace, North Range Group and Otapirian (Upper Triassic, Rhaetian) conglomerates. Rhyolites and dacites are the predominate volcanics in Kaihikuan (Middle Triassic, Ladinian) and Oretian (Upper Traissic, Carnian) conglomerates. Diorite, tonalite, and grandiorite pebbles are associated with the more basic volcanics whereas granites and adamellites are associated with the more, acid volcanics. Plutonic pebbles are most common in the Taringatura Group. This relationship, in addition to petrographic evidence, indicates that plutonic pebbles were derived from intrusive bodies underlying the volcanics.
Rudaceous-size material becomes generally finer grained from west to east in the Glenure Formation. Acid volcanic pebbles occur at a lower horizon in the eastern Hokonuis than in the western Hokonuis. The data indicate the source-area of the acid material is east of the source area of the more basic material.
Sandstones are usually massive, poorly sorted, and grading is not common. In many cases sands were probably transported by a grain flow process. Volcanic rock fragments or plagioclase predominates in the sandstones. Detrital quartz makes up <20% of the mode in all samples and in many cases is <5%.
The distribution of types of volcanic clasts in sandstones parallel those of conglomerate pebbles. In addition, Otamitan (Upper Triassic, Carnian) sandstones contain predominantly rhyolite and dacite fragments and Jurassic sandstones contain predominantly andesite fragments. The pattern is also verified by distribution of detrital clinopyroxene and Ti02 + Fe2o3 + FeO + MgO.
Sandstones with a color mottling on a scale of 3mm up to 1cm are widespread but are especially common in the Taringatura Group. The sandstones are frequently laumontite-rich but the mottled color is mainly due to the unequal distribution of chlorite cement.
Non-resistant siltstones are common throughout the sequence. Thick sequences of resistant siltstones occur mainly in the Taringatura Group. Resistant and non-resistant siltstones frequently contain animal trails and borings and traction current indicators, but grading is uncommon. The siltstones were probably deposited at similar depths to the sands but during times of minor tectonic activity. Many resistant siltstones contain albite and/or quartz cements.
Vitric and vitric-crystal tuffs are common throughout the sequence and have been altered mainly to heulandite, analcime, laumontite, and montmorillonite. The tuffs are believed to have been transported mainly by air and to have undergone little reworking. In several cases the deposits were prone to slumping. Graded tuffs are common in the Taringatura Group. Relict shards in tuffs of the Gavenwood Tuffs become progressively finer grained from west to east. Fresh pyroclastic feldspars in Oretian tuffs are more sodic than in Malakonian, Etalian and Otapirian tuffs.
Vein minerals in the area include laumontite, stilbite, analcime, heulandite group, albite, quartz, calcite, halloysite, and pumpellyite. Laumontite and stilbite are the most common types and occur mainly in pre-Oretian, Otapirian, and Jurassic rocks.
The distribution of secondary minerals is related to the bulk composition of the rocks. Secondary cements of prehnite, pumpellyite, laumontite and heulandite are restricted to sandstones containing predominantly andesitic clasts. High proportions of sphene and chlorite are also found in these samples. Albite and/or quartz cements are usually abundant in sandstones containing rhyolitic or dacitic clasts. Laumontite is abundant in Otapirian and Jurassic sandstones as well as in the North Range Group and pre-Fairplace beds. Prehnite is common in the North Range Group and occurs sporadically in Otapirian and Jurassic beds.
Plagioclase in fine sandstones ( < 0.2mm average grain size) is much less prone to albitization than in coarser sandstone from the same part of the sequence. Grains which are completely altered have a composition of Ab99.1 An0.9. Albitized portions of grains which are partially fresh have compositions significantly more calcic than completely albitized grains. There is a general tendency for increased albitization towards the base of the section if sandstones of similar grain-size are compared.
Laumontite occurs as a replacement of heulandite,detrital plagioclase and as a cement. Alteration of a heulanditized tuff to a laumontitized tuff is nearly an isochemical alteration except that MgO and Fe2O3 + FeO are significantly lower in the laumontitized tuff than in the heulanditized sample due to removal of phyllosilicates. The alteration has been controlled in part by jointing in the rocks.
Analcime pseudomorphs after heulandite are common and hence most analcime is believed to have been formed from heulandite group minerals. Analcime can be synthesized from heulandite group minerals at 100°c with solutions of NaOH (0.1m) and Na2CO3 (0.1M and 0.01M) in 3-week runs. Both increased pH and Na+ concentration favor the reaction. The Si/Al ratio of the analcime product is largely a function of the Si/Al ratio of the zeolite reactant. Analcime compositions in the Hokonuis are uniform throughout the sequence and appear to have equilibrated with quartz.
Heulandite group minerals in the Hokonuis typically replace volcanic glass or fills cavities in sandstones. They have a wide range of Si/Al ratios (3.0 to 4.3), are rich in Ca, have K/Na ratios '1 in most samples, and sometimes have appreciable Mg. Composition does not vary systematically with depth of burial. However, minerals with relatively high Si/Al ratios are particularly common in Oretian and Otamitan tuffs. Large crystals in cavities or glass vesicles of tuffs have lower Ca contents and higher Si/Al ratios than the crystals replacing glass shards. Composition of the heulandite group minerals is believed to have been controlled by the bulk composition of the glass.
Albite is a more common cement or cavity filling than quartz or K-feldspar. Albite replacement of heulandite has been documented in several samples.
Prehnite replaces plagioclase, fills cavities, and is inferred to have replaced heulandite in numerous samples. frequently with heulandite than with laumontite. Prehnite occurs more Two varieties are recognized: a "clear" type which contains 4.6 - 6.0 wt % total Fe as Fe2O3 and an inclusion riddled "spongy" type which contains 1.2 - 3.2 wt % total Fe as Fe2O3.
Pumpellyite usually occurs in the matrix but replaces plagioclase in some samples. Some of the pumpellyite is inferred to have replaced laumontite. The pumpellyite is optically negative and two optical orientations are recognized. The mineral contains up to 14 wt.% total Fe as Fe2O3.
Chlorites in the Hokonuis have wide variations in Al/Mg + Fe ratios (o.36- 0.56). Celadonites, which are most abundant in tuffs, have wide variations in Al/Fe ratios (0.51 - 2.80). Aluminous chlorite and aluminous celadonite co-exist in a tuff. EMA analyses are given for phases in tuffs which have compositions intermediate to chlorites and celadonites and may indicate an interlayered chlorite and celadonite.
Mixed-layer clay minerals, including both random and regular mixed-layer types, are not uncommon. Mixed layer montmorillonite-chlorite and montmorillonite-mica have been identified.
The geothermal gradient in the Hokonuis is estimated from mineral stability fields to be about 20°C/km. The mineral distribution pattern is interpreted in terms of variations in solution chemistry, initial rock composition, temperature, load pressure, and fluid pressure.
A general approach towards regional equilibrium is suggested by restricted zeolite mineralogy, absence of volcanic glass, and generally more advanced reconstitution· towards the base of the section, several examples of local disequilibrium are cited. However, several examples of local disequilibrium are cited.
Study of heulandite structural group. There is a full range of Si/Al ratios between heulandite and clinoptilolite end members. Ca, Na, and K can replace each other extensively, especially in clinoptilolites. Therefore, a new classification is proposed for heulandite group minerals based on Si/Al ratios. The subdivisions are made: heulandite (Si/Al < 4.00) and clinoptilolitG (Si/Al > 4.00). The dominant exchangeable cation in the unit cell is placed as a suffix to the subdivision e.g. heulandite-Ca.
If both loosely and tightly bound water are considered then + 0.6 H2O molecules are coordinated about divalent cations; and 3.2 + 0.5 molecules about monovalent cations in heulandite group minerals. The variation in density and refractive index of heulandite group minerals is largely a function of the variation in total water content.
Optical orientation of crystals mounted in Lakeside 70 is length fast if Si/Al < 3.52 and length slow if Si/Al E 3.57. Refractive index is strongly influenced by the types of cations present.
Comparison of calculated cell dimensions indicate (1) clinoptilolites typically have smaller a, c, and b parameters than heulandites (2) the area of the ac plane increases with increasing Al or divalent cation substitution; (3) b increases with increasing Mg substitution.
Unit cell dimensions are given for two different contracted phases, phase I and phase B of four dehydrated heulandites. For heulandite from Cape Blomidon, Nova Scotia, the spacing of the (020) reflection of the untreated mineral is 8.970A and for phase B is 8.30-8.35A. The phases appear at temperatures as low as 202°C~ 3°. For 11 heulandites and clinoptilolitesthe initial change to phase I occurs at f 213* to 312*C + 3* after heating for 2 hours and cooling for 1 hour.
Three types of thermal stability are recognized in the mineral group. Minerals with a sum of unit cell divalent cations equal to or greater than 1.87 contract after heating. Higher temperatures are required to contract samples with higher Si/Al ratios.
Composition of the heulandite structural group zeolites can be estimated using optical, cell dimension, and thermal stability data.
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Thesis description:
2 v. (406 leaves) : illus., diagrs., maps (7 col. fold. in pocket) plates (1 col) ; 30 cm.
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1971Boles
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Boles, James R. (James Richard), 1944-, “Stratigraphy, petrology, mineralogy, and metamorphism of mainly Triassic rocks, Hokonui Hills, Southland, New Zealand,” Otago Geology Theses, accessed October 5, 2024, https://theses.otagogeology.org.nz/items/show/49.