PRE-MIOCENE ROCKS AND STRUCTURES IN THE LANE MOUNTAIN-NORTH CALICO MOUNTAINS AREA, CENTRAL MOJAVE DESERT, CALIFORNIA

Geologic mapping and geochronologic data provide new insights into enigmatic pre-Miocene rocks in the Lane Mountain - North Calico Mountains north of Barstow, California. Geochronology includes zircon ages of plutonic rocks and detrital zircon ages from metasedimentary rocks.

About 8 km of tectonically assembled offshore metasedimentary and metavolcanic rocks are present, including meta-andesite, quartzite, argillite, conglomerate, hornfels, and carbonate rocks. Field relations and detrital zircon ages indicate that some of these rocks are Paleozoic but that many are Triassic and/or Jurassic. Some of the carbonate rocks resemble Paleozoic miogeoclinal units, and some of the youngest clastic rocks correlate with the Early Jurassic Fairview Valley Formation based on lithologic similarity and detrital zircons.

Intrusive rocks include Permo-Triassic (245-252 Ma) diorite/tonalite with Sri ~.704 Sri suggesting oceanic affinity, Late Jurassic (146-150 ma) diorite with Sri ~.705, 150 Ma leucocratic dikes and plugs, Late Cretaceous (81-84 ma) quartz monzonite with Sri ~.708, a felsite dike (84.5 Ma), and quartz monzonite porphyry dikes and plutons (73-80 Ma) with Sri ~.710 indicating continental basement. Undated Jurassic(?) gabbro is also present. All metamorphic rocks predate the Jurassic intrusive suite and some are intruded by the Permo-Triassic suite.

Pre-Miocene structural events include (D-1) tectonic assembly of Paleozoic metasedimentary and metavolcanic rocks followed by Permo-Triassic plutonism; (D-2) emplacement of miogeoclinal carbonates and folding, and (D-3) structural stacking and folding of Paleozoic and Early Mesozoic rocks before Late Jurassic plutonism; and (D4) emplacement of Late Jurassic diorite and folded Fairview Valley Formation pendants on a major Late Cretaceous or younger low-angle fault.

Regionally the pre-Miocene rocks form part of a sliver of oceanic-derived intrusive rocks and offshore metasedimentary rocks of Paleozoic and Mesozoic age sandwiched between Paleozoic and Mesozoic miogeoclinal rocks intruded by continental derived plutons of Permo-Triassic, Jurassic, and Cretaceous ages to the east and west. Major lateral and or stacked terrane boundaries have been proposed to explain the presence of this sliver but remain the subject of debate.