DETRITAL TOURMALINE IN THE ARCHEAN JARDINE METASEDIMENTARY SEQUENCE OF THE WESTERN BEARTOOTH MOUNTAINS, MT-WY, USA: EVIDENCE FOR HIGHLY EVOLVED ROCKS EXPOSED AT THE EARTH’S SURFACE AT 2.9-3.6 GA

The Jardine Metasedimentary Sequence (JMS) is a well-preserved, metamorphosed turbidite sequence deposited after 2.9 Ga (youngest detrital zircon) and before intrusion by granitoids at 2.8 Ga in the South Snowy Block of the western Beartooth Mountains in the northern Wyoming Province, USA. U-Pb ages of detrital zircons (2.9 to 3.6 Ga) and Sm-Nd depleted mantle model ages (3.2-4.0 Ga) suggest an ancient provenance for the protoliths, which included graywacke, quartzite, arkose, and shale. Sedimentary structures typically found in turbidite sequences are well preserved (e.g., graded beds, scour and fill, crossbedding) and suggest a mid-to-distal submarine fan on an active continental margin. Medium-grade, low-pressure metamorphism (550-600°C, 3-4 kbar) of metapelitic rocks commonly produces the diagnostic assemblage of staurolite-andalusite-biotite found in the JMS.

Within these rocks are well-developed metamorphic tourmalines, many with detrital cores. Based on tourmaline textures and compositions, metamorphic tourmaline began to crystallize in the JMS metaclastic rocks ~350°C and continued to grow along the prograde path to peak metamorphic conditions. More significantly, cores of pre-metamorphic detrital tourmaline are well preserved in the metamorphic tourmalines. These cores are distinguished from metamorphic overgrowths based on several features, including rounding, fracturing, distinctly different mineral inclusions, elemental zoning, and discrete discontinuities in chemical composition. Applying tourmaline provenance criteria of Henry and Guidotti (1985) and other distinctive elemental concentrations (e.g., Ca, Cr, Ti, and F), suggests the lithologic sources of these cores likely included peraluminous granitic rocks, weakly fractionated and oxidized granitic rocks, aluminous metapelites, Al-poor and Fe³⁺-rich metapsammites, magnesian metasediments, and calc-silicate rocks. Although it is uncertain whether the detrital tourmaline cores have experienced single or multiple sedimentary cycles, the compositions of these cores represent the erosional remnants of more evolved crust. Thus, the crust that formed during the 2.9-3.6 Ga interval contained a wider range of more evolved compositions than typically assumed for Mesoarchean and older crust.