The Greek Psalms often (always?) change out “rock” for something non-metaphorical when it refers to God. For instance, Psalm 18:46(27)a:
The LORD lives, and blessed be my rock (ṣûrî).
In the LXX (17:47) this becomes:
ζῇ κύριος, καὶ εὐλογητὸς ὁ θεός μου.
The Lord lives, and blessed be my God.
When ṣûr refers to God, it usually becomes θεός (God) in the LXX, less often βοηθὸς (helper) or ἀντιλήμπτωρ (supporter) rather than πέτρα (rock), the typical translation of ṣûr. The latter indeed is used in the Psalms when refers to a literal rock, e.g. 104(105):41, 26(27):5.
The fascinating exception is 60(61):3(2)b:
בְּצוּר־יָר֖וּם מִמֶּ֣נִּי תַנְחֵֽנִי
Lead me to the rock (ṣûr) that is higher than I
ἐν πέτρᾳ ὕψωσάς με
On a rock (πέτρα) you exalted me
There the meaning is apparently changed to avoid the equation of God and rock.
There is plenty of metaphorical language elsewhere in the Psalms, and usually it just seems to be translated into the LXX using normal equivalences. Is there some theological problem the LXX translator had with this metaphor?