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Paper No. 337-11
Presentation Time: 4:30 PM
THE HEARTBEAT OF AN ARC
Detrital zircons from northeast China record cyclic magmatism within a late Paleozoic Andean-style arc constructed along the Paleoasian margin of North China, one of the oldest crustal blocks on Earth. Paleozoic zircons from the arc record three magmatic pulses (HFEs) with a period of ∼60 m.y. that are reflected in zircon age spectra. Each pulse is accompanied by a secular negative εHf(t) excursion that represents increasing crustal melt contributions to arc magmatism and spans the full duration of the pulse. The intervening magmatic lulls are accompanied by rapid positive εHf(t) excursions that signify influxes of juvenile magma into the arc between each pulse. Each successive pulse attains elevated εHf(t) values relative to the last. The lack of similar isotopic patterns in zircons derived from contemporaneous intrusions into older continental blocks inboard of the arc indicate that this pattern was restricted to the arc itself. Magmatism in the arc terminated following the closure of the Paleoasian Ocean in Permo-Triassic time.
The antiquity of the North China crust, and the lack of significant magmatism within it for >1 b.y. prior to Paleozoic time, gives it an evolved epsilon Hf(t) character that differs greatly from juvenile mantle melt sources. The dataset presented here permits calculation of the Hf-isotopic character of the North China arc substrate during late Paleozoic time, which can then be compared to modeled values for depleted mantle during the same time period to determine the relative contributions of mantle vs. crustal melt sources over time within the evolving arc. The resulting time-series clearly shows the "heartbeat" of this arc, with the isotopic character of the arc magmas most likely driven by lithospheric thickening-foundering cycles having a period of 60 million years.