Diverse sources for igneous blocks in Franciscan melanges, California Coast Ranges

Journal Article · Thu Nov 01 00:00:00 EST 1990 · Journal of Geology; (USA)

DOI:https://doi.org/10.1086/629457· OSTI ID:5598819

MacPherson, G J ^[1]; Phipps, S P ^[2]; Grossman, J N ^[3]

---

1. Smithsonian Institution, Washington, DC (USA)
2. Univ. of Pennsylvania, Philadelphia (USA)
3. Geological Survey, Reston, VA (USA)

Igneous blocks in Franciscan melanges are of three chemical-petrologic types: (1) tholeiitic basalts of both arc and spreading center origin, with depletions in light relative to heavy rare-earth elements, 3% > TiO{sub 2} > 1%, high Y/Zr and Y/Ti ratios, and relict augites that generally have low Al and Ti and well-defined iron-enrichment trends; (2) basalts of probable seamount origin with marked enrichments in light relative to heavy rare-earth elements, 5% > TiO{sub 2} > 1%, lower Y/Zr and Y/Ti than (1), and Ti-Al-rich augites showing little if any iron-enrichment trends; and (3) hypabyssal intrusives having SiO{sub 2} > 52%, TiO{sub 2} < 1%, flat or only slightly fractionated rare-earth-abundance patterns, and diopsidic augites that are very low in Ti and Al and show no iron-enrichment trends. All of the blocks are metamorphosed; most are undeformed pumpellyite-bearing greenstones, and a few contain sodic amphibole {plus minus} lawsonite {plus minus} sodic pyroxene. The melanges are probably olistostromal in origin, deriving their igneous block detritus both from the downgoing Pacific plate (ocean floor basalts and seamounts) and from the hanging wall of the Franciscan trench (basalts and arc-related silic intrusive rocks). The silicic intrusive rocks and some of the basalts are eroded fragments of the fore-arc crust that ultimately become the Coast Range Ophiolite. These fragments were incorporated into the Franciscan trench fill and subducted. Results suggest that the igneous blocks in ophiolitic melanges provide important information about melange formation and about the tectonics and paleogeography of the regions in which the melanges are found.

• OSTI ID: 5598819
• Journal Information: Journal of Geology; (USA), Vol. 98:6; ISSN 0022-1376
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
CALIFORNIA
GEOLOGIC FORMATIONS
PETROLOGY
ALUMINIUM
BASALT
ENRICHMENT
GEOCHEMISTRY
GEOLOGIC HISTORY
GEOLOGY
IGNEOUS ROCKS
IRON
MESOZOIC ERA
METAMORPHIC ROCKS
ORIGIN
PLATE TECTONICS
QUANTITY RATIO
RARE EARTHS
SEDIMENTARY ROCKS
SILICON OXIDES
SUBDUCTION ZONES
TERTIARY PERIOD
TITANIUM
TITANIUM OXIDES
YTTRIUM
ZIRCONIUM
CENOZOIC ERA
CHALCOGENIDES
CHEMISTRY
DEVELOPED COUNTRIES
ELEMENTS
FEDERAL REGION IX
GEOLOGIC AGES
METALS
NORTH AMERICA
OXIDES
OXYGEN COMPOUNDS
ROCKS
SILICON COMPOUNDS
TECTONICS
TITANIUM COMPOUNDS
TRANSITION ELEMENT COMPOUNDS
TRANSITION ELEMENTS
USA
VOLCANIC ROCKS
580000* - Geosciences