This year we plan to have two field trips. Dates are still undecided and so is the detailed itinerary. The first trip will be a 1-day trip to Northern California, the second one a 3-hour trip into the Berkeley Hills in Kensington.
First field trip:
Probably Saturday October 11 to look at minerals and rocks in the California Coast Ranges with emphasis on the Franciscan Formation (High pressure/low temperature): Tiburon eclogite, Laytonville quarry (glaucophane, stilpnomelane, deerite, howieite, zussmanite etc.), Russian river
Requirement:
You will have to write a 2-page report on some mineralogical topic discussed and seen on the trip.
Second field trip:
Monday Nov. 24, 2 p.m.- 4 p.m.: Blake Gardens in Kensington - Lawson and Lawsonite.
Andrew Lawson (1861-1952) was one of the first geology professors at Berkeley. He was appointed byLeConte in 1890 and retired in 1928 but remained active until his death from pneumonia in 1952, shortly after his youngest son was born (in 1950). He taught the first field geology course in America, discovered the San Andreas fault and is known as a mineralogist. The mineral lawsonite is named after him. On the fieldtrip we will first visit one of the four houses Lawson built in Berkeley (1555 La Vereda, built in 1948 when he was 87). Afterwards we drive to Blake Garden in Kensington where we will see Fransciscan minerals lawsonite, glaucophane, actinolite, pumpellyite, albite, and calcite among others.
FRANCISCAN ROCKS IN THE BAY AREA
The Franciscan Complex is mainly of Upper-Jurassic-Cretaceous age. It is composed of graywackes, dark shales, mafic volcanic rocks of submarine extrusive origin (pillow basalts), chert, some limestones and locally unusual metamorphic rocks with mineral assemblages indicating high pressure and low temperature during crystallization. In addition there are some complexes of altered ultramafic rocks (e.g., serpentine) which may represent relics of mantle material.
The metamorphic assemblages are quite unique and obeserved in only a few places on Earth. As a Berkeley student you should be familiar with these rocks. Franciscan rocks outcrop in large parts of the California Coastal Ranges, including Berkeley back yards.
Much of the Franciscan consists of blocks of various metamorphic rock types floating within a pervasively sheared matrix (shale or less commonly serpentine). This mixture is called a "mélange". Other parts of the Franciscan Complex consist of stratigraphically coherent units that, on a map scale, are cut by abundant faults, resulting in a highly disrupted terrane of fault-bounded slivers. Some rocks (eg. limestones at Laytonville) had their origin far away on the Southern hemisphere and were accreted to the North American continent. It is assumed that a Late Jurassic trench off the coast was filled with sediments which descended with the oceanic plate underneath the North American continent. (Fig. 1) Mineral assemblages (eg. jadeite, glaucophane, aragonite) suggest subduction to 20-30 km (~8, k bars). Before thermal equilibrium was established (heat conduction is low, whereas pressure effects are immediate), the mass was uplifted again to the surface and juxtaposed with nonmetamorphic rocks. Temperature pressure conditions can be estimated from equilibrium phase diagrams (Fig. 2,4).
One of the high pressure minerals is lawsonite, name after Andrew C. Lawson, who taught the first field class in America in the spring of 1891, in the Berkeley Hills (see the picture in 367 McCone). Three other interesting minerals (discovered in 1965 by Stuart Agrell from Cambridge University in the Berkeley mineral collection) are deerite, howieite and zussmanite. The outcrop in the Laytonville quarry which we will be our first stop. Below is a brief summary of some typical Franciscan rocks and minerals.
ROCKS
Sedimentary rocks:
graywacke: feldspar and chlorite-rich sandstone.
chert: silica-rich, fine-grained rock.
Igneous and metamorphic rocks:
serpentinite: mainly serpentine--Mg3(OH)4[Si2O5]--either igneous, or metavolcanic
5Mg2[SiO4] + 4H2O --> 2Mg3(OH)4[Si2O5] + 4MgO + SiO2
forsterite _____________________ serpentine
(in peridotites)2MgCa(CO3)2 + 2SiO2 --> 2CaCO3 + Mg2[SiO4] + 2CO2
__ dolomite________________ calcite _________forsteriteeclogite: omphacite(pyroxene)-pyrope (garnet) rock; small amount of quartz; indicator of high pressure, high temperature. Upper mantle may be composed largely of eclogites.
blueschists: glaucophane, riebeckite ("blue amphiboles").
MINERALS
Sulfides:
pyrite: FeS2. Cube.
cinnabar: HgS. Hydrothermal.
Carbonates:
aragonite: CaCO3. High pressure polymorph of calcite.
Silicates:
pyroxenes: Single chain of SiO4 tetrahedra.
aegirine: NaFe3+Si2O6
jadeite: NaAlSi2O6
omphacite (Ca, Na)(Mg, Fe2+, Fe3+, Al)Si2O6
amphiboles: Double chains of SiO4 tetrahedra
grunerite: (Fe2+)4(Fe2+, Mg)3[Si8O22](OH)2
riebeckite (Na equivalent of actinolite): Na2Fe32+Fe23+[Si8O22](OH)2
(crocidolite: fibrous riebeckite)
glaucophane (Na equivalent of tremolite): Na2Mg3Al2[Si8O22](OH)2
orthosilicates
lawsonite: CaAl2(OH)2[Si2O7]H2O
epidote: Ca2Fe3+Al2OˇOH[Si2O7][SiO4]
clinozoisite: Ca2AlˇAl2OˇOH[Si2O7][SiO4]
pumpellyite: Ca4MgAl5O(OH)3[Si2O7][SiO4]ˇ2H20
garnet: pyrope: Mg3Al2Si3O12
spessartine: Mn3Al2Si3O12
sheet silicates
stilpnomelane: (K, Ca, Na)2(Fe, Mg, Al)29(OH)16[Si32O92]ˇ13H2O
grenalite: Fe62+Si4O10(OH)8 (chlorite)
cronstedtite: (Fe42+Fe23+)(Si2Fe23+)O10(OH)8 (chlorite)
phengite: K2Al2Mg2Si8O10(OH)4 (muscovite with Si:Al>3)
framework silicates
albite: NaAlSi3O8
anorthite: CaAl2Si2O8
new minerals from Laytonville
deerite: Fe122+Fe63+[Si12O40](OH)10
zussmanite: KFe132+Si17AlO42(OH)4
howieite: Na(Fe2+, Mn)10(Fe3+Al)2[Si12O31](OH)13
IMPORTANT METAMORPHIC REACTIONS (formulate the chemical equations)
calcite --> argonite
albite --> jadeite + quartz
labradorite (albite 50%, anorthite 50%) + olivine + diopside --> omphacite + garnet + quartz
albite + antigorite --> glaucophane + water
albite + tremolite + chlorite --> glaucophane + lawsonite
Last updated: 08/07/08