MAGMATIC PROCESSES IN THE CENTRAL SIERRA NEVADA BATHOLITH: A CRYPTIC PRE-JURASSIC BOUNDARY IN LONG VALLEY, CA

Oxygen isotopes are well suited for tracing input of supracrustal rocks into magmas during granite genesis at convergent margins. We present d¹⁸O values for zircon (Zc) from 155 granitic rocks across the Mariposa 1 x 2 ° sheet in the central Sierra Nevada batholith (SNB), whose varying age and composition allow us to track the evolution of the d¹⁸O across the batholith.

Comparison of d¹⁸O(Zc) to d¹⁸O quartz (Qt), and whole rock (WR) show Zc is most suitable for studying d¹⁸O because: (1) post-magmatic alteration has reset d¹⁸O(Qt) ~ 0.5‰ (2) Differing modes, plus crystal fractionation and alteration create variable d¹⁸O(WR) that obscures trends in magmatic d¹⁸O.

Variation of d¹⁸O(Zc) across the batholith (projected onto a SW to NE transect) reveals that high values (7.0 - 8.5‰) dominate the westernmost part of the batholith and decrease monotonically toward the Sr_i=0.706 isopleth. A 0.5‰ drop in d¹⁸O at the 0.706 line is resolvable with Zc but not WR. East of the 0.706 line, d¹⁸O(Zc) values (except Shaver Lake suite) decrease further (6.75 - 5.75‰) toward the Sierra Crest and increase sharply by 1‰ at the eastern SNB margin.

The patterns of d¹⁸O(Zc) allow us to reconsider why the d¹⁸O-Sr_i trends in the SNB do not match the remarkable d¹⁸O(WR) vs. Sr_i trends in the coeval Peninsular Ranges batholith in southern CA. Values of d¹⁸O(Zc) rule out alteration as causing the scattered d¹⁸O-Sr_i correlations, and thus no single two-component mixing model applies to the entire batholith. For example, the Fine Gold suite in the western part of the study area, with high-d¹⁸O, low-Sr_i values, records substantial input of hydrothermally altered, mantle-derived rocks, such as subducted ocean crust or volcanic arc sediments. The high-d¹⁸O, high-Sr_i values in the Shaver Lake suite records contamination by Proterozoic metasediments of the Kings Sequence. Values of d¹⁸O(Zc) that approach mantle-like values in the Eastern SNB, including Late Cretaceous Sierra Crest plutons, are consistent with a lithospheric mantle signature as is proposed for the Sierra Crest plutons (e.g., Coleman and Glazner, 1997, Int Geol Rev).

Finally, the observed sharp 1‰ increase of d¹⁸O(Zc) at the eastern SNB margin is recorded in Cretaceous, Jurassic, and Triassic rocks in Long Valley and the northern White Mountains. The occurrence of this d¹⁸O increase of rocks of these ages defines a major pre-Jurassic crustal boundary.