APATITE TRACE ELEMENT VARIATION AND INSIGHTS INTO MAGMATISM IN THE TUOLUMNE INTRUSIVE COMPLEX (TIC)

Apatite separates from eight samples of the TIC were analyzed for major and trace elements by EMP and LA-ICPMS, respectively. All major units are represented, from outer (oldest) to inner (youngest): Kuna Crest (KC), equigranular Half Dome (eHD), porphyritic Half Dome (pHD), Cathedral Peak (CP), and Johnson granite (JG). Magmatism spanned ~10 m.y (ca 95 to 86 Ma).

Apatite is F-rich, (<0.08 Cl, <0.05 S apfu); F increases inward so that CP and JG grains are fully fluorinated. In general, apatite trace element data from KC and eHD samples plot in distinct arrays, whereas JG, CP, and pHD apatite compositions plot in a single, sublinear trend: a feature also seen in hornblende data. For example, Sr contents are nearly constant in KC (300–350 ppm) and eHD (ca. 200 ppm) apatite, but increase by ca. 500 ppm from pHD to JG. For the suite, U contents decrease from KC to JG (95–10ppm), although Th varies slightly except in eHD apatite, where cores are enriched compared to rims. Rare rounded cores in pHD apatite are also Th-rich and considered to be antecrysts from the eHD. LREE in KC and eHD apatite decrease with decreasing U; however, in younger units LREE increase inward from pHD to JP. In contrast, HREE abundances broadly decrease from KC to JG.

The presence of distinct compositional trends in apatite from the KC and eHD is consistent with emplacement of compositionally variable magmas that cannot be related by fractional crystallization [1]. Abrupt core-to-rim decreases in Th and LREE in the eHD sample may reflect mixing, but are more likely to mark precipitation of titanite ± allanite. The transition from individual trends to a single trend among apatite from pHD, CP, and JG samples suggests that as the system evolved, magma compositions became more uniform. Nevertheless, this uniformity is inconsistent with fractional crystallization or with down-temperature increases in K(d), because compatible elements such as Sr, Y, and LREE increase with differentiation when they should decrease [2]. Instead, we suggest that while major element compositions are buffered by multiply-saturated source(s), trace elements such as Sr, Y, and LREE were enriched either by episodic mixing or by H₂O-fluxed rejuvenation of a successively more refractory source.

[1] Am.Min. 101, p.328. [2] CMP 166, p. 1; J.Pet. 55, p.1619.