Late Cretaceous Plutonic and Metamorphic Overprint of Proterozoic Metasediments of Ontario Ridge, Eastern San Gabriel Mountains, California
Bibliographic record
Abstract
The rifting of Rodinia, particularly in western Laurentia, has been a longstanding geologic problem, as Proterozoic rocks in the western U.S. are few and far between. The eastern San Gabriel Mountains contain a large metasedimentary package that has never been dated or thoroughly mapped. We present U-Pb detrital zircon geochronology results of 439 grains dated at CSUN???s LA-ICPMS; also 24 plutonic grains and 19 metamorphic rims of detrital grains analyzed on Stanford???s SHRIMP-RG, along with a detailed map, cross section and stratigraphic column for these Ontario Ridge metasediments. Probability plots of 206Pb/207Pb ages for five quartzite samples show three distinct peaks at ~1200, 1380-1470 and 1740-1780 Ma. Maximum depositional age is constrained by 3 grains between 906 to 934 Ma, which are 11-28% discordant. The ~1200 Ma peak distinguishes the Ontario Ridge quartzites, and is rarely seen in the western U.S. There are a few minor possible western sources, or, due in part to similarities with the Big Bear Group (Barth and Wooden, 2009), the rocks may represent sediment shed from Rodinia???s conjugate rift pair with western Laurentia. (Barth and Wooden, 2009). The detrital age signature also appears to match preliminary data from the Potato Mountain block (Premo et al., 2007), displaced ~8 km from the main metasediment body by the left-lateral San Antonio Canyon fault. The metasediments are intruded by pre-metamorphic granodiorite of Icehouse Canyon (85.9??0.6 Ma) and post-metamorphic quartz diorite of Ontario Peak (75.8??0.9 Ma). Two quartzites, metamorphosed to upper-amphibolite facies, contain zircons with \nmetamorphic rims (75.7??1.2 Ma: 12 grains and 76.7??2.4 Ma: 7 grains), consistent with the last documented plutonic and metamorphic event in the San Gabriel Mountains. These new data provide important keys for future research into the Proterozoic and Mesozoic history of southwestern North America, including the breakup of Rodinia.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.006 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".