DECODING THE TIMING WITHIN THE PAPAROA METAMORPHIC CORE COMPLEX, NEW ZEALAND: NEW INSIGHTS OF THE BREAK-UP OF EASTERN GONDWANA

The Paparoa metamorphic core complex (PCC) in New Zealand is located within the W-Province of Zealandia as part of eastern Gondwana and indicates the breakup of the supercontinent. The understanding of timing within the complex gives insight to the breakup of Gondwana, development of the Tasman Sea and its tectonic setting. We have used in situ isotopic and trace element analysis of zircons in combination of high precision CA-TIMS geochronology on 6 samples to emphasize complex zircon behavior during extension and exhumation of the PCC.

Within the samples we can distinguish between two distinctly different zircon populations: The most common Type A can be found in all 6 samples are elongate doubly terminating grains, commonly having oscillatory zoning within CL images and are interpreted as igneous. In contrast Type B zircons which only occur in the samples within the northern detachment plane are characterized by a smaller grain size. Also the grains have a similar morphology the crystal surfaces have a spongy occurrence or in extreme cases seem to be partly dissolved. In CL these grains are lacking clear and sharp features. Some small patches of remnant primary magmatic zoning are visible,. Featureless areas have inward penetrating lobate edges that embay the primary domains.

Incompatible elements display clear differences. Type A zircons are characterized by strong positive Ce anomalies (Ce/Ce* = 4-66.8), negative Eu anomalies (Eu/Eu* = 0.1-0.41) and SmN/LaN ratios ranging between 2 and 463. Type B zircons are characterized by low positive Ce anomalies (Ce/Ce* = 1.2-2.0), variable Eu anomalies (Eu/Eu* = 0.09-0.67), SmN/LaN ratios smaller than 5.

We were able to separate the two types of zircon temporally as well. The Type A Zircons range between 110 and 109 Ma whereas Type B Zircons cluster generally 1 Ma younger at 108 Ma.

After careful consideration of zircon growth mechanisms we interpret the Type A Zircons as magmatic indicating the emplacement of several continuous phases of the Buckland Granite within the PCC from 110-109 Ma whereas Type B Zircons have been modified in a fluid-rich environment, possibly associated with the initial movement along the northern detachment of the PCC roughly 1 Ma later. These new results would move the onset of the breakup of Zealandia from Gondwana ~10 Ma earlier than previous postulated.