RECOGNIZING DISTINCT MAGMAS AND DISCERNING MAGMATIC PROCESSES USING HORNBLENDE TRACE ELEMENT VARIABILITY FROM THE TUOLUMNE INTRUSIVE COMPLEX

Intracrystalline trace element variability in hornblende (Hbl) from the Kuna Crest Lobe (KCL) of the Tuolumne Intrusive Complex (TIC) shows distinct compositional arrays between each lithological unit in the lobe [1]. These arrays cannot be related through fractional crystallization alone, suggesting that individual batches of magma can be recognized. This approach was extended to Hbl from several locations within the interior of the TIC, including samples from the Kuna Crest (KC), equigranular Half Dome (eHD), and porphyritic Half Dome (pHD) units. Amphiboles from the TIC commonly display patchy zoning ranging from olive colored magnesiohornblende with Si (apfu) < 7.48 to blueish-green or colorless actinolite with Si (apfu) > 7.56. Olive colored zones are interpreted to be magmatic, with crystallization temperatures ranging from 697­-824°C [2]. Blueish-green–colorless actinolite yields temperatures < 694°C, indicating solidus and/or subsolidus recrystallization. Similar to Hbl from the KCL [1], elements such as Ti, Zr, Hf, Sr, Ba, Nb, Ta, and REEs generally decrease from crystal interiors to rims, supporting compatibility during Hbl crystallization. Hornblende from each unit (KC, eHD, pHD) defines distinct compositional trends in plots of Zr vs Y, Nb, Ta, and REEs. For example, Y, Nb, Ta, and the REEs show wide ranges of values over narrow ranges of Zr in KC samples, whereas in eHD samples these elements decrease regularly with decreasing Zr, and in pHD Hbl they remain constant. This variability suggests that sphene crystallization played an important role in the trace element variability of KC hornblendes, even at high temperatures. Furthermore, pHD magmas must have either been separated from melts where sphene was a residual phase or pHD magmas were already saturated in sphene prior to Hbl crystallization. Ba and Sr decrease with decreasing Zr, exhibiting broad overlap, although individual samples may follow distinct trends. The data show the KC, eHD, and pHD units to be compositionally distinct from one another. Moreover, while Hbl compositions from individual samples within each unit show broad overlap, they also show distinct compositional trends, suggesting that construction of each TIC unit involved multiple, similar but distinct magma batches. [1] Barnes et al., 2016 Am. Min.; [2] Putirka, 2016 Am. Min.