The Geochronology of the Plutonic and Metamorphic Rocks of New Zealand

Geochemistry and Geology

Aronson, James Louis ; Wasserburg, Gerald J.

University:California Institute of Technology

Department:Geological and Planetary Sciences

关键词: Geochemistry and Geology;   

Others  :  https://thesis.library.caltech.edu/9610/1/Aronson_jl_1966.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

Extensive Rubidium-Strontium age determinations on both mineraland total rock samples of the crystalline rocks of New Zealand, whichalmost solely crop out in the South Island, indicate widespread plutonicand metamorphic activity occurred during two periods, one about 100-118 million years ago and the other about 340-370 million years ago.The former results date the Rangitata Orogeny as Cretaceous. Theyassociate extensive plutonic activity with this orogeny which upliftedand metamorphosed the rocks of the New Zealand Geosyncline,although no field association between the metamorphosed geosynclinalrocks and plutonic rocks has been found. The Cretaceous plutonicrocks occur to the west in the Foreland Province in Fiordland, Nelson,and Westland, geographically separated from the GeosynclinalProvince. Because of this synchronous timing of plutonic and highpressure metamorphic activity in spatially separated belts, the RangitataOrogeny in New Zealand is very similar to late Mesozoic orogenicactivity in many other areas of the circum-Pacific margin (Miyashiro,1961).

The 340-370 million year rocks, both plutonic and metamorphic,have been found only in that part of the Foreland Province north of theAlpine Fault. There, they are concentrated along the west coast over a distance of 500 km, and appear scattered inland from the coast.Probably this activity marks the outstanding Phanerozoicstratigraphic gap in New Zealand which occurred after the LowerDevonian.

A few crystalline rocks in the Foreland Province north of theAlpine Fault with measured ages intermediate between 340 and 120million years have been found. Of these, those with more than onemineral examined give discordant results. All of these rocks aretentatively regarded as 340-370 million year old rocks that have beenvariously disturbed during the Rangitata Orogeny, 100-120 millionyears ago.

In addition to these two periods, plutonic activity, dominantlybasic and ultrabasic, but including the development of some rocks ofintermediate and acidic composition, occurred along the margin of theGeosynclinal Province at its border with the Foreland Province duringPermian times about 245 million years ago, and this activity possiblyextended into the Mesozoic.

Evidence from rubidium-strontium analyses of minerals and atotal rock, and from uranium, thorium, and lead analyses of uniformeuhedral zircons from a meta-igneous portion of the CharlestonGneiss, previously mapped as Precambrian, indicate that this rockis a 350-370 million year old plutonic rock metamorphosed 100 millionyea rs ago during the Rangitata Orogeny. No crystalline rocks withprimary Precambrian ages have been found in New Zealand. However,Pb²⁰⁷/Pb₂₀₆ ages of 1360 million years and 1370 million years havebeen determined for rounded detrital zircons separated from each oftwo hornfels samples of one of New Zealand's olde st sedimentary units,the Greenland Series. These two samples were metamorphosed 345-370 million years ago. They occur along the west coast, north of theAlpine Fault, at Waitaha River and Moeraki River, separated by 135km. The Precambrian measured ages are most likely minimum agesfor the oldest source area which provided the detrital zircons becausethe uranium, thorium and lead data are highly discordant. Theseresults are of fundamental importance for the tectonic picture of theSouthwest Pacific margin and demonstrate the existence of relativelyold continental crust of some lateral extent in the neighborhood of NewZealand.