UNVEILING MAGMATIC HISTORY IN THE ASH: USING BENTONITES AS A TIME SENSITIVE PROXY FOR THE EMPLACEMENT OF THE IDAHO BATHOLITH

Bentonites are layers of volcanic tephra that get deposited and diagenetically altered to pure claystone in the stratigraphic record. Because of their volcanic origin, bentonites contain a robust record of geochemical information inherited from the producing plutonic complex. To test the extent of isotopic preservation, a series of bentonites from the Bighorn Basin were sampled through a stratigraphic section representing nearly 10 million years of magmatism, from the Albian Thermopolis Shale through the Cenomanian Mowry and Frontier Formations. This 10-million-year period coincides with the major emplacement of magmatism in the Idaho batholith, the region that most likely produced the voluminous ash beds of this region.

To better understand the influence that crustal assimilation has on tephra, the Sr and Nd isotopic composition was determined for 48 bentonites from the Bighorn Basin region of northwestern Wyoming, south of Billings, MT. These data were used in conjunction with U-Pb geochronology of magmatic zircon grains sieved from each bentonite bed in order to put the samples into a precise temporal context. The Sr composition (⁸⁷Sr/⁸⁶Sr) of the samples shows a rapid transition during the Late Albian, from 0.705 at ~105 Ma, to 0.7066 at ~104.5 Ma, to 0.7083 at ~103 Ma. Within 2 million years of magmatism, the volcanic ash preserved the record of magmatic migration across the isotopic gradient into the Laurentian crust of the Proterozoic Selway terrane, marking the initiation of the main stage of the Idaho batholith volcanism, as recorded in the igneous complex. The Sr composition of plutonic rocks from Idaho show a rapid transition across the 0.706 isopleth, from 0.704 – 0.706 west of the Salmon River Suture Zone, to 0.706 – 0.708+ east of this boundary into primarily Precambrian Laurentian crust. The plutonic emplacement migrated eastward through the Late Albian into the Early Cenomanian and includes the lateral movement across this isotopic boundary. Stratigraphically, this coincides with the deposition of the Albian-aged Thermopolis shale and Cenomanian-aged Mowry Shale, respectively. Utilizing Sr isotopic compositions of tephra can be an effective way to track magmatism through slices of geologic time and may be an important tool to track ancient magmatic systems that lack preserved, unmetamorphosed plutonic systems.