Continuous metamorphic gradient documented by graphitization and K-Ar age,southeast Otago, New Zealand

The Chrystalls Beach-Brighten coastal section, southwest of Dunedin, New Ze aland, exposes Otago Schist, including the weakly metamorphosed Triassic ac cretionary melange of the Chrystalls Beach Complex and higher-grade rocks o f a largely felsic, are-derived provenance. Melange zones contain pelagic s ediments, some of which are manganiferous, and metabasites. The grade of me tamorphism increases progressively from southwest to northeast. Four minera l zones are recognized largely on the basis of mineral assemblages in psamm itic and semipelitic rocks: pumpellyite-chlorite, pumpellyite-actinolite, e pidote-actinolite, and biotite. d(002) data for carbonaceous material, b(0) values for phengite, and metamorphic phengite K-Ar ages are reported for 3 2 pelitic and semipelitic samples. Remarkably tight correlations are reveal ed between d(002) for progressively graphitized carbonaceous material, K-Ar ages that range from 184.6 to 138.7 Ma, and linear distance along the sect ion. Both the d(002) values and the K-Ar ages decrease progressively toward the northeast through the four mineral zones and with progressive textural changes. All rocks cropping out in the section have undergone a coherent e pisode of progressive metamorphism associated with terrane collision and ha ve not been disturbed by any later major displacements. A convex trend for phengite bo values plotted against advancing graphitization suggests PIT co nditions during metamorphism as in the high-pressure intermediate facies se ries. The age of the metamorphic peak is inferred to be 175-155 Ma (Middle to Late Jurassic), with cooling to the mica closure temperature at 155-135 Ma (Late Jurassic to Early Cretaceous). Unloading, denudation, and cooling were accompanied by localized hydrothermal events. The oldest covering stra ta were deposited at ca. 100 Ma and denudation was completed by ca. 75-80 M a. Middle Triassic radiolarian nodules in melange zones are dated as preced ing the metamorphic peak by about 65 Ma. Associated turbidites contain susp ected late Middle to early Late Triassic tube fossils.