The given schematic diagram shows different stages of development of S\(_2\) foliation after the formation of S\(_1\) foliation. Which of the following statements CORRECTLY interpret(s) the relationships of porphyroblasts W, X, Y and Z with the matrix?
Show Hint
A porphyroblast that traps S1 and is wrapped by S2 grew before/during S2; one with no trapped fabric that overprints S2 grew after it, that is post-tectonic.
'X' and 'Z' are post-tectonic to S\(_2\) foliation
The sequence of porphyroblast growth is W → Y → X and Z
Show Solution
The Correct Option isC
Solution and Explanation
Step 1: Set up the two textures to look for in the sketch.
In a porphyroblast matrix diagram like this, there are really only two textures that matter for timing. Texture one: a grain with curved trails of an older fabric ($S_1$) locked inside it, around which the younger fabric ($S_2$) flows smoothly, meaning the grain was already there when $S_2$ formed. Texture two: a blank grain with no trapped fabric at all, sitting across the $S_2$ trend and breaking it, meaning the grain grew only after $S_2$ was already laid down.
Step 2: Sort W, X, Y, Z into these two textures.
W and Y both carry the $S_1$ label on curved internal trails, and the surrounding $S_2$ lines bend around them without crossing through, so both belong to texture one, pre to syn $S_2$. X and Z show no internal trails at all, just plain shading, and the $S_2$ lines of the matrix stop against or wrap awkwardly around their edges rather than flowing through, so both belong to texture two, post $S_2$.
Step 3: Go through the four statements with this sorting in hand.
Statement A says W grew early within the $S_2$ event, but W's texture places its growth before $S_2$ started, not during an early part of it, so A does not hold. Statement B claims W came after Y specifically, yet W and Y show identical textures with nothing in the diagram to rank one ahead of the other, so B is not supported. Statement D repeats this same unsupported ranking of W before Y, so it also fails.
Step 4: Check statement C against the sorting.
Statement C says X and Z are post-tectonic to $S_2$, which is exactly texture two as identified in Step 2 for both grains. This one holds up.
Step 5: Conclude.
Only C, that X and Z are post-tectonic to $S_2$ foliation, is correct.
\[ \boxed{\text{Only option C: X and Z are post-tectonic to } S_2 \text{ foliation}} \]
