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
Verified By Collegedunia
The Correct Option isC
Solution and Explanation
Step 1: Recall how porphyroblast timing is read from a diagram like this.
When a mineral grain grows in a rock that is being deformed, its relation to the surrounding foliation tells us when it grew. A grain that traps a straight or gently curved older fabric (here \(S_1\)) inside it, while the younger matrix fabric (\(S_2\)) wraps smoothly around its outside without cutting through it, grew before or during the deformation that made \(S_2\), it is pre to syn tectonic with respect to \(S_2\). A grain with no trapped internal fabric at all, whose outline instead truncates or overprints the \(S_2\) trend running through the matrix, grew after \(S_2\) had already formed, it is post tectonic to \(S_2\).
Step 2: Read porphyroblasts W and Y in the figure.
Both W and Y show internal trails labelled \(S_1\), curved fabric preserved from the earlier deformation, and both sit inside a matrix whose \(S_2\) foliation bends smoothly around their outline rather than cutting across them. This means W and Y both grew with the older \(S_1\) fabric already trapped inside them, before the \(S_2\) matrix fabric had fully formed, so they are pre to syn tectonic with respect to \(S_2\). Nothing in the figure orders W relative to Y, both simply preserve \(S_1\) and are wrapped by \(S_2\) in the same way.
Step 3: Read porphyroblasts X and Z in the figure.
X and Z are shown as plain, unpatterned grains with no internal foliation trapped inside them, sitting discordant to the \(S_2\) trend of the matrix, which stops at or is overgrown by their outline instead of bending smoothly around it. This is the signature of growth after the \(S_2\) fabric was already established, so X and Z are post-tectonic to \(S_2\) foliation.
Step 4: Analyze option (A).
"'W' formed at an early stage of \(S_2\) foliation" implies W grew during the early part of the \(S_2\) forming event. But W preserves \(S_1\) trapped inside it and is passively wrapped by \(S_2\), which places its growth before \(S_2\) began, not at an early stage within the \(S_2\) event itself. Incorrect.
Step 5: Analyze option (B).
"'W' formed after the growth of porphyroblast 'Y'" claims a specific order between W and Y. The figure shows both with the same relationship to \(S_1\) and \(S_2\), it gives no basis for saying one grew after the other. Incorrect.
Step 6: Analyze option (C).
"'X' and 'Z' are post-tectonic to \(S_2\) foliation" matches the reading in Step 3 exactly, both grains lack trapped \(S_1\) fabric and overprint the \(S_2\) trend. Correct.
Step 7: Analyze option (D).
"The sequence of porphyroblast growth is W \(\rightarrow\) Y \(\rightarrow\) X and Z" asserts a strict order with W before Y. As found in Step 2, the figure cannot distinguish W's timing from Y's, it only shows that both predate X and Z. So this specific sequence is not supported. Incorrect.
Step 8: Final conclusion.
Only the statement that X and Z are post-tectonic to \(S_2\) foliation is correct.
\[
\boxed{\text{X and Z are post-tectonic to } S_2 \text{ foliation (option C)}}
\]
