MET LABS 3 and 4: METABASITES

EESC 3000: Igneous and Metamorphic Petrology

Learning Objectives:  Students will improve their ability to describe a metamorphic rock  Students will be able to assign metamorphic facies to metabasite samples  Students will be able to use a petrogenetic grid to interpret pressure and temperature conditions of metamorphic rocks  Students will be able to draw isograds on a geological map based on mineral assemblage data

Key Minerals: Actinolite, Albite, Augite (CPX), Biotite, Chlorite, Epidote, Garnet, Glaucophane, Lawsonite, Omphacite, Orthopyroxene, Plagioclase, Prehnite, Pumpellyite, Rutile, Titanite, Quartz, Zeolites

In Lab 2 you described thin-sections 670 and CAZ-1, and in Lecture 2 we determined the rock names and PT conditions associated with these two rocks. In Labs 3 and 4 you will describe the following six additional metabasite samples following the same procedures outlined in Lab 2. AMYG: Pomperaug Basin, CT CT1: O and G Quarry, Southbury, CT PROSPECT: Watchung basalt, Passaic County, NJ W409: Locality Unknown W437: Locality Unknown Z3A: Locality Unknown In addition, you will determine the IUGS-recommended names (see flowchart from Lecture 2), metamorphic facies, and peak metamorphic conditions (see attached facies assemblage table and petrogenetic grids) of the six new metabasite samples. When you have completed petrographic descriptions of all 6 samples, use these data to draw facies-delineating isograds on the attached map. Based on your isograd patterns, infer the likely mineral assemblages that would be found is drill-core samples DC1 and DC2. Using your isograd map, and the data in the table of metamorphic facies and their associated mineral assemblages, infer mineral assemblages that would be likely to occur in metapelite samples MP1 through MP4. Record these mineral assemblages in the blanks below the map.

Due at Beginning of Lab 4: Completed table of properties for minerals in metabasites; full descriptions of any two samples from AMYG, CT1, PROSPECT, W409, W437, Z3A, including name and metamorphic facies; summary of GeoRef and/or Mindat research results for the three samples for which the provenance is known. Submit a copy of the Petrogenetic Grid for Metabasites showing the PT field over which the mineral assemblages in each of the two samples are stable. (Use the in-class example of 670 and CAZ-1 as a model.)

1 Due at Beginning of Lab 5: Full descriptions of the remaining four samples, including name and metamorphic facies. Submit a copy of the Petrogenetic Grid for Metabasites showing the PT field over which the mineral assemblages in each of the four samples are stable. Submit the completed isograd map. ______

Research Results (Attach printout of results pages.)

Sample #: AMYG Locality: Pomperaug Basin, CT

Website Source: GeoRef Mindat (circle one) Metamorphic Rock and Mineral Keywords Cited: ______

Sample #: CT1 Locality: O and G Quarry, Southbury, CT

Website Source: GeoRef Mindat (circle one) Metamorphic Rock and Mineral Keywords Cited: ______

Sample #: PROSPECT Locality: Watchung basalt, Passaic County, NJ

Website Source: GeoRef Mindat (circle one) Metamorphic Rock and Mineral Keywords Cited: ______

2 Summary of Minerals in Metabasites

Mineral Diagnostic Features Similar Minerals

Actinolite

Albite

Augite (CPX)

Biotite

Chlorite

Epidote

Garnet

Glaucophane

Lawsonite

Omphacite

Orthopyroxene

Plagioclase

Prehnite

Pumpellyite

Rutile

Titanite

Quartz

Zeolites

3 4 Metamorphic Facies and their Characteristic Minerals and Mineral Assemblages

FACIES Metabasites Metapelites Marbles* Metaperidotites* Zeolite Zeolites such as laumontite and Mixed-layer clays (XRD) Cc-Do-Qz Srp (Chrysotile) heulandite etc. (in place of other Ca- Al silicates such as Prh, Pm, and Ep) Subgreenschist Prh-Pm, Pm-Ac, Prh-Act (Prh and Illite(XRD)-Chl-Ab-Qz Cc-Do-Qz Srp (Chrysotile) Pm are the diagnostic Ca-Al silicates rather than minerals of the epidote or zeolite groups) Greenschist Ac-Ep-Chl-Ab (an epidote group Chl-Ms-Ab (low T) Cc-Do-Qz Brucite-Srp, Srp-Fo mineral is the diagnostic Ca-Al Bt-Chl-Ms-Ab (high T) silicate rather than Prh or Pm) Epidote-Amphibolite Hb-Ab-Ep(-Chl) Gt-Bt-Chl-Ms-Ab Amphibolite Hb-Pl St-Als-Ms (Low T) Cc-Do-Tr, Fo-Tr-Srp, Fo-Tlc, (plagioclase more calcic than An17) Sil-Kf-Ms(-Gt or Cd) Cc-Do-Di, Fo-Tr-En, En-Tr-Qz Sil-Gt-Cd (no Kf) (High T) Cc-Do-Fo Granulite Cpx-Opx-Pl (Ol not stable with Pl or Cd-Gt-Kf-Sil (mod P), with Gt) Ky-Kf (high P) Pyroxene Hornfels Cpx-Opx-Pl (Ol stable with Pl) Cd-And-Kf Cc-Fo, Di-Gt-Wo Anthophyllite-Fo Sanadinite Distinguished from pxn hornfels Corundum-An-Mt (no Qz) Wo-An-Di facies by especially high-T minerals (e.g. pigeonite, K-rich labradorite) Blueschist Gl-Ep(-Gt), Gl-Lw(-Jd) Ms-Gt-(Tc or Chl) (no Bt) Aragonite Eclogite Omphacite-Gt-Qz (no Pl or Ol stable Tc-Ky(-Gt-Ms) with Gt)

* Mineral assemblages in marbles and metaperidotites are dependent upon XCO2 and so may deviate from metamorphic facies scheme.