Rock Identification

  
Rock Identification
Each number is located over or points to a specific rock unit:
»  Identify the rock type (igneous, sedimentary, metamorphic) and specific rock type (e.g. Greenschist, arkose, etc.) that would reasonably be found at each location.
»  Describe the processes by which each rock formed. If the rock is altered from a parent identify the parent.
»  Identify the tectonic regime and/or Wilson Cycle stage each rock formed in.

Each number is hot; clicking on it will take you to an identification and interpretation of the rock at the numbered tectonic location.
 
 
 
1 - Plagiogranites and Alkaligranites. Plutonic (phaneritic/batholithic) Felsic igneous rocks of the original continent. Although plagiogranites and alkaligranites would be common, other felsic rocks such as Syenites and Monzonites would be expected here too.

2 - Lithic rich sediments deposited in a foreland Basin and derived from the erosion of the (Stage E) volcanic arc that collided with Westcontinent inStage F. Sediment is in the green field on QFL Diagram and is lithic rich because the volcanic arc initially can supply only volcanic rock fragments. Later in its erosion it can supply feldspars too (note how the green field extends toward the feldspar side of the QFL) as the Diorite and Granodiorite batholiths are exposed.
     Quartz is typically low (<50%) because intermediate igneous rocks are Quartz poor, although Plagiogranites and Granodiorites which are not uncommon in volcanic arcs may supply some. quartz also may come from metamorphic processes. 

3 - Quartz Sandstone - veneer of clean Quartz beach sand deposited on Westcontinent after the Stage B rifting as the continental terrace subsided below sea level (Detailed Cross Section) and the sea Transgressed across the continental edge (yellow field on QFL).

4 - Arkosic sediments (QFL; Orange Field) deposited in the half graben formed to the sides of the axial rift (Cross Section). Because the rock lifted up in the horsts is feldspar rich Felsic continental basement the sediment will also be feldspar rich, i.e. arkose.

5 - Carbonate sedimentary rocks. Divergent Continental Margin sediments, here Carbonates, but they could be clastic sediments if the sourceland and climte favor them. They are formed on the rifted, subsiding eastern margin of Westcontinent after the rifting of Stages B and C. The DCM will continue to develop in tectonic quiescence until something collides with it.

6 - Mafic (Basalt) or felsic (Rhyolite) volcanic rocks; this is a rift volcano, and can be either mafic or felsic composition depending on conditions . The composition will be mafic if the volcano is built from the hot spot magmas derived from the mantle. It will be felsic if it forms from magma generated by fractional melting of the lower continental crust. This occurs when the hot spot magma ponds at the base of the continent (See Cross Section).

7 - Mixed Mafic (Basalt) and Felsic (Granite) Transition crust - continental Granites massively invaded by mafic columnar jointed dikes during the first stages of ocean crust Formation in a rifting event. (Cross Section).

8 - Pillow Basalts and Gabbros. This is mafic igneous rocks of the oceanic lithosphere (the upper two layers of the Ophiolite Suite). At this locality the oceanic lithospheric rocks have been subducted under the volcanic arc.

9 - Quartz/Lithic rich sedimentary rocks (QFL, Blue Field) deposited in the foreland Basin formed by the collision of Westcontinent and Eastcontinent during Stage H. 

10 - Metamorphic Blueschist Melange. Derived from high pressure/low temperature metamorphism of the lithic rich sediments and accompanying igneous rocks deposited in and above the subduction zone trench of the Stage E volcanic arc. 

11 - Quartz Sandstone; Stage I. This is the veneer of sediments deposited on the peneplained continent after the Stage H continent-continent collision orogeny. The Quartz sand is the only thing remaining from the erosion of all the mountains in this Cycle, because it is the only mineral resistant to weathering.
      However, it is possible for Shale to be present on the edges of the continent, especially in the early history of the peneplain, and Carbonate deposition may be common in tropical climates when sea level is high enough to put part of the continent under water.

12 - DioriteGranodiorite, Plagiogranite; intermediate igneous rocks. These are the batholiths formed above the Stage E subduction zone in the core of the volcanic arc. They are generated by the Fractional Melting of the mafic igneous rocks above the subducting oceanic slab.

13 - Granulite. High grade (granulate facies) Barrovian metamorphism of a mafic parent rock next to the Stage E volcanic arc batholiths. The mafic parents are the rocks of the ocean lithosphere (Ophiolite Suite).

14 - Amphibolite. Middle grade (Amphibolite facies) Barrovian metamorphism of a mafic parent rock in the vicinity of the Stage E volcanic arc batholiths. The mafic parent are the rocks of the ocean lithosphere (Ophiolite Suite).

15 - Greenschist. Low grade (Greenschist facies) Barrovian metamorphism of a mafic parent rock in the outer edges of the Stage E volcanic arc batholiths. The mafic parent are the rocks of the ocean lithosphere (Ophiolite Suite).

16 - Metamorphic Blueschist Melange. High pressure/low temperature metamorphism of sediments deposited in and above the subduction zone trench of the Stage G cordilleran orogeny. 

17 - Slates and Phyllites. These are old DCM sediments of Eastcontinent (see right side of Stage F) which have undergone low grade (Greenschist facies) Barrovian metamorphism during the Stage G cordillera orogeny of that continent.

18 - Schists. These are old DCM sediments of Eastcontinent (see right side of Stage F) which have undergone middle grade (Amphibolite facies) Barrovian metamorphism during the Stage G cordilleran orogeny of that continent.

19 - Gneiss and Migmatites. These are old DCM sediments of Eastcontinent (see right side of Stage F) which have undergone high grade (granulate facies) Barrovian metamorphism during the Stage G cordillera orogeny of that continent.

20 - Diorites and other intermediate plutonic rocks, including Plagiogranites. These are the batholiths formed above the Stage G cordilleran subduction zone under the western margin of Eastcontinent. They are generated by the fractional melting of mafic igneous rocks above the subducting oceanic slab.

21 - Arkosic sediments. This is a backarc Basin formed by tensional rifting and graben Formation. Because the basement rock being brought to the surface is felsic igneous continental basement we would expect these to be arkoses (Orange Field On The QFL). However, if sediments derived from the erosion of the mountains (Stage H) were to wash in here the sediments would be very Quartz rich with a fair percent of lithics. The sediment would probably plot in the blue field on the QFL Diagram.

22 - Plagiogranites, Alkaligranites and other felsic igneous rocks. Plutonic (phaneritic/batholithic) felsic igneous rocks of the original continent.

23 - Pillow Basalts and Gabbros; mafic igneous rocks of the oceanic crust (Ophiolite Suite).

24 - Dunites and peridotites; ultramafic igneous rocks. These are mantle rocks below the Moho. 

25 - Basalts and Gabbros; mafic igneous rocks. These are hotspot derived magmas rising out of the mantle and penetrating through the continent in the back arc region. They are derived from the small convection cell that rises in the back arc region above the subducting slab.
 
Wednesday, July 24, 2013
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