A U-PB AND LU-HF ISOTOPIC STUDY OF ARCHEAN ROCKS FROM THE MINNESOTA RIVER VALLEY: EVIDENCE FOR THE MINNESOTA AND SACRED HEART OROGENIES

The Archean Minnesota River Valley (MRV) subprovince is composed of four granitic gneiss terranes that are defined primarily on gravity and magnetic survey data. The Morton and Montevideo blocks have the best exposures and it has long been understood that these are some of the oldest rocks on Earth. Modern U-Pb zircon SIMS and TIMS data show a long and complicated history for these rocks with primary crystallization at ca. 3500 Ma followed by multiple zircon forming events until ca. 2600 Ma. This study adds LA-ICPMS U-Pb and Hf zircon data from three samples of the Morton and Montevideo gneisses and one sample of the Grt-Bt-Opx paragneiss at Granite Falls. A tonalitic sample from the Morton gneiss yields an upper intercept age of 3530 ± 33 Ma and a granitic sample from the same outcrop yields an upper intercept age of 3370 ± 32 Ma, both interpreted to be crystallization ages. A granitic gneiss sample from the Montevideo block yields an upper intercept crystallization age of 3503 ± 36 Ma. All three samples from the two gneisses show an almost continuous range of concordant dates that span ages from 3500 to 2600 Ma. The Montevideo gneiss sample has inherited grains, with one zircon yielding a discordant age of ca. 3900 Ma. The paragneiss yielded 276 dates, with concordant ages ranging from 2588 to 3049 Ma and most of the dates clustering at 2671 ± 27 Ma (244 grains, MSWD=2.73). A younger cluster (9 grains) shows an upper intercept date of 2586 ± 30 Ma. The zircons from this sample are rounded and appear to be detrital. We interpret the prominent peak at 2671 to be the maximum depositional age of these rocks that were shed from the Minnesota Orogeny and the younger cluster of ages near 2600 Ma to be a metamorphic age associated with the Sacred Heart Orogeny.

εHf values from the Morton samples range from juvenile to evolved and are bracketed by crustal evolution curves that project back to depleted mantle model ages of 3600 to 3800 Ma. The Montevideo gneiss is similar but also contains young zircons (2600-2800 Ma) with intermediate to juvenile signatures suggesting mixing of evolved and more primitive materials. εHf values from the 2671 Ma prominent peak of the paragneiss range from intermediate to primitive and support a detrital origin for these zircons.