Geology of Timaru Creek and Junction Creek

Gach, Carina M., 1967-

Geological mapping of the Timaru and Junction Creek area deals with mainly psammitic schist interbedded with greenschists, pelitic schist and metaconglomerates. One unconformity on a regional scale was determined. Metacherts and manganese pods were found as river boulders (up to lm in diameter) but never in situ., but their occurrence corresponds to squences dominated by pelitic schists. Polyphase deformation of the schist was investigated using mesoscopic structures. The results were compatible with the work done by Norris and Cooper (1977), although only two deformation phases were found during this investigation. Strain-estimations for the ductile deformation of the mapped rocks were derived from investigations of conglomeratic schist using the Rf/<j>-Method. "Pan-cake" like deformed quartzofeldspathic clasts were shortened about 88% along the Z-axis. Norris and Bishop (1990) suggested for more than 70% TZIII thus this value characterise the Timaru and Junction Creek area schists as TZrrr. Petrographic work on the greywacke schists of the Timaru and Junction Creek area revealed that the whole mapped area is characterised by pumpellyiteactinolite facies metamorphism, at least 4.8km across strike perpendicular to the foliation (20-60° constantley to the east). For a geothermal gradient of l8°C/km and a constant mC02/mH20 the temperature difference of 70°C and the pressure difference of 1kb would exist over such an extend of the pumpellyite-actinolite facies. The interpretation of the petrological and structural data of the mapped area would suggest a formation model similar to that described by Ward and Sporli (1979). Their explanation for steeply dipping foliation and subhorizontal isograds, and therefore for the wide extent of the pumpellyite-actinolite facies rocks, includes large-scale recumbent folds. They suggest that foliation does not form simultaneous with the isograds they are formed during later burial regional metamorphism. Because no evidence for these large scale recumbent folds was found in the mapped area, the author suggests isoclinal folding. These were formed due to progressive steepening and folding of successive thrust sheets, comparable to the structures seen in modern accretionary prisms at convergent margins, is the alternative suggestion of the author. The main part of this project was the determination of the mineralogy of the manganese pods, found as river boulders all along Junction Creek. This included microprobe analyses of four thin sections of different manganese pods iii I '' (_. -~ --~ -- --'-----~--~----· --· _. -- and three of their contact material. Identification of, two of the minerals was verified by X-ray diffraction. The mineral association consists of friedelite, caryopilite, rhodonite, rhodochrosite, axinite, spessartine and a brown brittle micaceous mineral.. Minor amounts of MnS and a manganeous arsenic cobaltite (CoAsS) also occur. Geochemical investigation was made of four greenschist samples and on two samples of the manganese pods. The results of plotting discriminant data in several diagrams is ambiguous and raises the question wether such diagrams are appropriate for metabasites that may have undergone substantial element mobility during metamorphism. Because of the restricted number of samples the only conclusion possible is that the samples plot in fields outlined by previous studies of metabasites of various metamorphic grades. Comparison of the manganese pods with analyses of manganese nodules given by different authors suggest two possible origins for the manganese association of Timaru and Junction Creek samples. Either the manganese pods are sea-floor manganese nodules or they have formed hydrothermally. The association of these manganese pods with the turbidite sequences indicate a marine formation. The interpretation of the petrological and structural data of the mapped area would suggest a formation model similar to that described by Ward and Sporli (1979). Their explanation for steeply dipping foliation and subhorizontal isograds, and thereJore for the wide extent of the pumpellyite-actinolite fades rocks, includes large-scale recumbent folds. Instead of these large scale recumbent folds, isoclinal folding due to progressive steepening and folding of successive thrust sheets, comparable to the structures seen in modern accretionary prisms at convergent margins, is the alternative suggestion of the author.

v, 105 p. : ill. (some col.), maps (some col.) ; 30 cm.

1997Gach

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